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Elucidating the structure of the graphitic carbon nitride nanomaterials via X-ray photoelectron spectroscopy and X-ray powder diffraction techniques

Introduction

A meteoric career of graphitic carbon nitride (gCN-C 3 N 4 ) as a solar-driven, metal-free photocatalyst began in 2009 with the paper of Wang and Antonietti. 1 In the age of increasing environmental awareness, it hit on the fertile ground opening a new window of opportunities for the development of a cheap method for hydrogen production using visible light. [2][3][4] As time went by, it became evident that the photocatalytic activity of gCN is insufficient, mainly because of the fast recombination of electron-hole pairs. Several attempts have been made to improve gCN properties by its doping with heteroatoms, [5][6][7][8][9] synthesis of nanostructural carbon nitrides with unique morphology and large surface area, [10][11][12][13][14] the formation of nanocomposites, [15][16][17] and subjecting to exfoliation. [18][19][20][21][22] Many of these attempts resulted in improved activity, however, these methods are labor-intensive and expensive which causes their application on a commercial scale is doomed to failure. Still, the simplest and the cheapest method of graphitic carbon nitride synthesis is the thermal condensation of cheap and easily available materials such as urea, thiourea, cyanamide, dicyandiamide, and melamine. [23][24][25][26][27][28][29] Carbon nitride obtained by the above-mentioned method was characterized many times but despite this, controversies concerning its structure and composition still exist. This was pointed out by Miller et al. 30,31 and Kessler et al., 32 who expressed the opinion that it is unjustified to use the term "graphitic carbon nitride" for a wide range of materials of diverse compositions. A vast majority of materials described by the formula C 3 N 4 are, in fact, microcrystalline materials containing hydrogen in their structure and should be described by the formula C x N y H z or called Liebig's melon with the formula C 2 N 3 H. The aforementioned authors have proposed to label these materials as gCN and we followed this proposition in our work. Also the structure of gCN arouses many controversies. Models based on s-triazine and tri-s-triazine (heptazine) were proposed. [33][34][35] Theoretical considerations showed that the tri-s-triazine-based model is energetically favored compared to that based on striazine subunit as it was confirmed by DFT studies. [33][34][35] Heptazine subunits can combine via tertiary nitrogen into fully condensed structure (Fig. 1a) which forms layers set one above the other just as in a graphite material (Fig. 1c). Another Please do not adjust margins Please do not adjust margins possibility consists in combining the heptazine subunits into melon chains (Fig. 1b) which also form layers through hydrogen interactions (Fig. 1d). The discrimination between both types of structures is difficult. Thus development of new methods for the exact structure determination is of high importance.

Herein we conducted detailed XPS and XRD studies of the graphitic carbon nitride derived by thermal condensation of dicyandiamide to prove that these methods can be easily employed to describe and confirm the exact structure of the graphitic carbon nitride. We showed that the method of thermal condensation, used in this work, enable to obtain, irrespective of temperature, carbon nitride built of melon chains and not fully-condensed graphitic carbon nitride.

Fig. 1 The structure of graphitic carbon nitride. a) and c) represent fully condensed structure; and b) and d) melon-type structure (according to Ref. 36 ).

Experimental Section

Preparation of graphitic carbon nitride

Graphitic carbon nitride was prepared by the pyrolysis of dicyandiamide (DCDA) (Sigma-Aldrich, 99%) in a semi-closed system according to a defined procedure. Typically, 4 g of DCDA precursor was put into a quartz crucible (50 mL) with a cover and heated at 500, 550, or 600 °C in a muffle furnace for 4h with a heating rate of 10 °C•min -1 under ambient pressure in the air.

The crucible was allowed to remain in the furnace until it cooled down to room temperature. The obtained yellow material was ground into a fine powder in an agate mortar. The powder was stored in a desiccator over molecular sieves 4A. The samples were labeled with codes D-500, D-550, and D-600, where the number designates synthesis temperature in o C.

X-ray photoelectron spectroscopy (XPS)

The XPS study was aimed at analyzing chemical bonding and elemental composition. A Kratos Axis Ultra spectrometer (Kratos Analytical, UK) was used. The base pressure during spectra acquisition was better than 10 -6 Pa and was achieved using an ion pump with cryoshroud (220 L•s -1 ). The excitation source was monochromatized aluminum K radiation (1486.6 eV) operated at 10 mA and 15 kV. The recorded spectra included wide and narrow scans and the total acquisition time was approximately 1 hour. The spectra were acquired sequentially. The calibration and linearity of the binding energy scale were confirmed and charge stabilization was achieved by using the Kratos Axis device. With the selected scan parameters, the full width at half maximum (FWHM) of the Ag 3d 5/2 peak of a standard silver sample was about 0.9 eV. The size of the analyzed sample area was about 700 μm x 300 μm. The sample powders were pressed into small stainless steel troughs mounted on a multi-specimen holder, before loading into the spectrometer. The transfer procedure within the spectrometer included exposure to the pumping system for 10 hours before conducting the XPS analyses. The pass energy of the hemispherical analyzer was set at 160 eV for the wide scan and 40 eV for narrow scans. The charge referencing method used was the C(C, H) component of the C 1s peak of adventitious carbon fixed at 284.6 eV. The data treatment was performed using CasaXPS software (Casa Software Ltd., UK). Molar fractions were calculated using peak areas normalized on the Please do not adjust margins Please do not adjust margins basis of acquisition parameters after a Shirley background subtraction and corrected with experimental sensitivity factors and transmission factors provided by the manufacturer. The description of the XPS procedure was based on the guidelines from the paper by Greczynski and Hultman. 37

X-ray powder diffraction (XRD)

The powder X-ray diffraction patterns of the samples were obtained using a Bruker D8 Advance diffractometer with Gemonochromatized CuK radiation (λ = 1.5406 Å). The XRD data were collected over a 2-theta range of 5 -80° with a step size of 0.01° and the scanning time rate of 5 seconds. The parameters describing the diffractograms and the unit cell parameter values were determined using Winplotr and UnitCell programs, respectively. 38,39 The structure models used for the refinement of gCN were placed in the Electronic Supplementary Material (ESI).

Results and discussion

Structure analysis by X-ray photoelectron spectroscopy (XPS)

The XPS study can provide valuable information on the structure of carbon nitride (both qualitative and quantitative one) and about the content of respective elements and their forms. The interpretation is, however, difficult because of the ambiguity of assignment of respective signals to specific species, particularly concerning the N 1s of nitrogen. [START_REF] Broitman | Tribology of Diamond-Like Carbon Films: Fundamentals and Applications[END_REF] In the case of C 1s signal, there is quite a high agreement to assign the signal at 288. Also, the some authors 47 suggested that the shoulder observed at 399.3 eV can origin from the nitrogen in NH 2 group. The above ascription was confirmed many times in later papers (Table 1).

Table 1 N 1s core level binding energies for graphitic carbon nitride. The respective forms of nitrogen were numbered according to increasing binding energy sequence: N1, N2, N3, N4. In parentheses, the difference in BE was shown between a given band and N 1s band of pyridinic nitrogen (N1).

* Our results

Absolute values of binding energies of respective signals depend on the value assumed during the spectra calibration, whereas differences in BE (the values are shown in parentheses in Table 1) between a respective band and the pyridinic nitrogen Py(N1) band are characterized by a good agreement. This lends credence to the assumed interpretation and signal ascription.

The study of Akaike et al. 47 (Table 1, entry 8) seems to dissipate the controversy concerning the assignment of bands. The authors proved that the signal appearing at the highest binding energy in the N 1s range (around 401.5 eV) originates from the N(C) 3 component (quaternary nitrogen -Q). 47 Moreover, they have found that the signals originating from primary and secondary amines can be split and that binding energy of primary amines is lower than that of secondary amines. The assignment of bands was summarized in Figure 2. The respective forms of nitrogen were numbered according to the increasing binding energy sequence: N1, N2, N3, N4. Analysis of data presented in Table 1 and conclusions drawn by Akaike et al. 47 induced us to accept this method for the interpretation of XPS spectra measured in our work.

The survey spectrum of D-600 (Fig. 3a) showed that the sample was composed solely of C, N, and O, which may have originated from adsorbed water molecules. 15,36,54,55 Fig. 2 The binding energy of N 1s signal in the XPS spectra and assignment to specific nitrogen signals. The C 1s XPS spectra of the D-600 sample (Fig. 3b) contained two dominant components located at 284.6 and 288.2 eV, corresponding to graphitic carbon (C[C,H] adventitious carbon, AdC), and NCN in the triazine or heptazine rings, respectively. To get a better fitting of the spectra, it was necessary to introduce the third signal with a maximum at 286.9 eV. In the case of the D-600 sample, its contribution to the N 1s signal is 0.7 %, in that of the D-550 sample it is somewhat less (0.4 %) and the D-500 sample does not contain it at all (see Table 2). In general, this signal is greater in the case of the samples synthesized at higher temperatures. It can be thus ascribed to nitrile species CN. [56][57][58][59][60] Another nitrile species signal visible in N 1s region is at 400.2 eV (it will be discussed later). Dante et al. 61 observed nitrile species formation during the calcination of carbon nitride at high temperatures. They claim that the thermal degradation of the graphitic carbon nitride occurs through cyano group formation. The evolution of HCN already at 570-600 o C was reported also in some cases. 43 The synthesis temperatures used in this study (550 and 600 o C) and a long time of annealing (4 h) are sufficient for the thermal degradation of a part of heptazine rings to cyano groups. This supposition is justified by the fact that they were not observed in the case of the sample synthesized at 500 o C and that their quantity rises with increasing the calcination temperature.

The N 1s spectrum has a complex structure with a clear signal at 398.7 eV and there exists a consensus on attributing it to pyridinic nitrogen in the heptazine (or triazine) ring (N1) -Fig. 3c. A clear shoulder at higher energies can be fitted using three curves having maxima at 399.4, 400.3, and 401.3 eV. They originate from primary amine (N2), secondary amine (N3), and quaternary nitrogen (N4), respectively. The percentage of respective forms of nitrogen (based on XPS spectra) has been presented in Table 2. The ratios of N1 : N2 : N3 : N4 for respective samples are as follows: A clear rise in pyridinic nitrogen (N1) and a decrease in amino (N2) and imino (N3) nitrogen are observed with increasing temperature.

Py(N1) NH 2 (N2) NH(N3) Q(N4) D-
The obtained percentages of different forms of nitrogen should reflect atomic ratios in the real structure of carbon nitride. Can we reach such a conclusion and what are the contributions of respective forms of nitrogen to the structure of carbon nitride?

The percentage of respective forms of nitrogen can be calculated by creating simple geometric models of carbon nitride consisting of heptazine subunits. In Table 3, models are presented of carbon nitrides based on the melon structure (the linear form of carbon nitride) and the fully condensed gCN (the network structure) as well as formulae with the use of which the number of atoms of a respective kind of nitrogen can be calculated. The melon structure can be regarded either as a single chain with increasing length (Msc -melon single-chain model) or as a model composed of many chains (Mmc -melon multi-chain model). It was assumed that in the network model (Hn -heptazine network), the number of heptazine subunits in X and Y directions is the same. The formulae presented in Table 3 enabled to calculate percentages of respective forms of nitrogen (N1, N2, N3, N4) in the different carbon nitride structures of increasing size (Table 3 and Fig. 4). Percentages of nitrogen forms in the samples synthesized from dicyandiamide at 500, 550, and 600 o C, as determined from the XPS studies, are presented in Fig. 4. Theoretical calculations [33][34][35] show that the most stable form of carbon nitride is the structure composed of tri-s-triazine (heptazine) units, therefore we may eliminate the triazine network model from our considerations. A frequently suggested carbon nitride structure is the fully condensed heptazine network (Hn). It results from the geometric model, which was assumed by us, that pyridinic nitrogen content (N1) should range from 66.7 to 71.7 % (beginning from the smallest to the greatest structures). The structures consisting of 10 subunits contain 70 % nitrogen N1. However, the percentage of nitrogen N1 in the studied samples is considerably lower and increases with rising temperature in order 57.5, 62.5, 64.8 % (Table 2). This discrepancy indicates that the Hn model not necessarily describes correctly the structure obtained by us. The quaternary nitrogen (N4) content also undermines it. In the Hn model, it should range between 13.9-19.9 % (Table 3), whereas in our samples it is lower, namely from 8.2 to 9.1 % (Table 2). Moreover, the theoretical content of amino nitrogen, according to the Hn model should be 13.9% for the structures consisting of 4 subunits and 4.8% for the structures composed of 30 heptazine subunits (Table 3).

The XPS measurements show the values ranging from 15.2-22.2 % (Table 2), which are considerably higher than those predicted by the Hn model. This means that the measured contents of the respective forms of nitrogen are out of tune with the theoretical predictions for the model of fully condensed carbon nitride.

A better fitting of experimental data was observed by using models based on the chain structure of melon -melon singlechain (Msc) and melon multi-chain (Mmc). The later reflects the real composition of samples particularly well. The real content of nitrogen N1 perfectly fits into the Mmc model -the XPS Please do not adjust margins Please do not adjust margins results almost exactly lie on the theoretical curve (Fig. 4). A similar agreement occurs for amino nitrogen N2. The real contents of nitrogen N4 are somewhat lower than those predicted by the Mmc model (by about 1-2 %). In turn, there is somewhat more nitrogen N3 (NH groups) than predicted by the theoretical model. The increased content of nitrogen N3 can be elucidated by the presence of nitrile species CN which were observed in the C 1s spectrum and which give a signal at 400.1 eV in the N 1s spectrum. 62 The small content of CN groups in the samples and the proximity of the position to that of the peak from NH (400.5 eV) caused that they are indistinguishable and reckoned to the NH (N3) peak and this explains the observed higher content of nitrogen N3 in the samples. It worth to remind that nitrile groups can be formed at high temperatures by the decomposition of heptazine rings 61 which in turn, reduces the amount of quaternary nitrogen (N4).

A comparison of the real content of respective forms of nitrogen in carbon nitride samples to theoretical predictions shows that only melon multi-chain correctly reflects the quantitative experimental data obtained from the XPS measurements. It should be assumed that the carbon nitride formed is a linear melon-type form rather than a fully condensed network structure (Hn). The above considerations bring to a general conclusion that the interpretation of XPS spectra should not be limited to its qualitative analysis. Quantitative analysis of respective nitrogen forms should always be conducted and only on this basis a final conclusion on carbon nitride structure can be drawn.

The determination of crystal structure by X-ray powder diffraction (XRD)

The crystal structure of 3D graphitic carbon nitride arouses many controversies. In the literature, two main models were reported. One of them is based on the structure of fully polymerized polyheptazine (Fig. 5a) and the other one, on partially polymerized, where heptazine subunits form poly(aminoimino)heptazine chains (called melon) (Fig. 5b). The structure of fully-condensed carbon nitride is described by the hexagonal cell 63,64 and space group P6m2, 65 whereas the linear structure of melon-type by the orthorhombic cell and space group P2 1 2 1 2 proposed by Lotsch et al. 36 Fina et al. 66 and by Seyfarth et al. 67 In Fig. 6 the X-ray diffraction pattern of carbon nitride prepared by us at 600 o C was juxtaposed with theoretical models calculated for unit cells P6m2 and P2 1 2 1 2 and intensities and positions of reflections determined theoretically for ideal unit cells of hexagonal P6m2 (187) and orthorhombic P2 1 2 1 2 (18) systems.

Many authors point at the hexagonal model because it results from the basic information provided by the powder diffraction pattern with an annular alternate arrangement of carbon and nitrogen atoms and the formation of layers connected by van der Waals interactions. It creates opportune conditions for the formation of more and more number of symmetry elements, which translates into a smaller number of reflections in the XRD spectrum. However, even a superficial analysis of Fig. 6 allows to conclude that the orthorombic system for the melon-type graphitic carbon nitride (model A) better describes the structure of the studied material than the hexagonal system. In B model, peaks at 10.8, 17.7 21.9 2 are missing which explicitly indicates the occurrence of an order lower than hexagonal. Please do not adjust margins Please do not adjust margins The XRD patterns of gCN samples obtained at different temperatures are presented in Fig. 7. All gCN samples on the powder diffraction spectra contain several broad reflections that confirm their nanocrystalline (or even amorphous) nature.

All the XRD patterns have a major strong peak at about 27.5° 2 which is indicative of a layered component with an interlayer spacing of 3.2 Å (Fig. 7). With a rise in the synthesis temperature, the peak at 27.5° 2 becomes more and more intense and sharper. It shifts to greater 2 angles, which indicates an increase in the system order; the crystallites become larger and the interplanar spacing d decreases from 3.254 Å do 3.215 Å (see Table 4). This indicates an increase in the number of layers parallel to the 002 plane as a result of the continuous condensation process of carbon nitride chains with increasing temperature until the system becomes stable (Table 4). A confirmation of this phenomenon is a small but noticeable increase in the size of crystallites in the direction of the c axis of the unit cell as determined using the Scherrer equation (Table 4). 68 The aforementioned rise in the size of crystallites in the direction of the c axis means that the number of condensed layers increases. Graphitic carbon nitride synthesized at 500 o C consists of 22 layers, whereas that synthesized at 600 o C has 31 layers. The above considerations about the size of crystallites and number of condensed layers should be regarded as approximate because there exists an additional factor influencing the relative reduction in the intensity of hk0 reflections in the powder diffraction pattern of the gCN structure, namely the translation of flat defects in the neighbouring layers which causes the displacement of layers. These displacements are responsible for the increase in the disorder of 3D structure in compliance with the model proposed by Seyfarth et al. 67 and Lotsch et al. 36 The low angle signal at about 13 o 2 corresponds to the distance d = 6.8 Å and can be indexed as the reflection (210) which refers to the distance between two adjacent melon chains in the crystal (Fig. 5b). 36,47,66,69 Also in this case, a gradual decrease in the d-spacing with increasing condensation temperature (from 6.857 to 6.797 Åsee Table 4) is observed. As shown above, X-ray diffraction patterns of carbon nitride samples obtained by the thermal condensation of dicyandiamide can be described correctly only by assuming the orthorombic system with the space group P2 1 2 1 2 and presuming that the layers are formed from the chains of poly(aminoimino)heptazine (melon). The results are consistent with those drawn from the XPS study.

Summary

The paper presents results of structural studies of graphitic carbon nitrides obtained by the thermal condensation of dicyandiamide in a semi-closed system with the access of air. This method is the most frequently used, the simplest and the cheapest for the synthesis of graphitic carbon nitride.

It results from this study that carbon nitride prepared by the thermal condensation consists of melon chains as suggested earlier by Miller et al. 30 and Kessler et al., 32 rather than fully condensed polyheptazine structure. This is testified by the relative ratios of pyridinic, amino, imino, and quaternary nitrogen obtained from XPS analysis. These ratios correspond to the melon structure and are out of tune with the fully condensed structure. Such a conclusion is also confirmed by the XRD results which proved that the 3D crystal structure of carbon nitride can be correctly described only by the orthorombic cell and space group P2 1 2 1 2 applied to condensed chains of poly(aminoimino)heptazine (melon). Both XPS and XRD studies have shown that a rise in the synthesis temperature results in the increase in the condensation degree and the degree of structural order of carbon nitride. The interplanar distance d decreases systematically with the rise in the synthesis temperature and at the same time the number of condensed layers increases. Moreover, it was shown that the XPS investigation should not be limited to qualitative studies. Only quantitative XPS analysis allows us to correctly identify the carbon nitride structure.

2 ( 4 )

 24 eV to sp 2 -bonded carbon (NCN) in the triazine or heptazine rings and the signal at 284.6(8) eV to graphitic carbon (C[C,H] adventitious carbon, AdC) which usually serves for energy calibration.[START_REF] Zhang | [END_REF] On the other hand, the interpretation of nitrogen signals N 1s arouses controversy. Usually, three main signals observed at binding energies (BE) of 398.6, 399.7, 401.1 eV are ascribed to pyridinic nitrogen (denoted here as Py), tertiary nitrogen N(C) 3 , i.e. the central nitrogen of the heptazine ring (also called quaternary; denoted as Q) and amino NH 2 or NH or pyrrolic (denoted as NH x ) groups, respectively.16,[START_REF] Zhang | [END_REF][42][43][44][45] There are also other interpretations according to which the signals at 399.7 and 401.1 eV are ascribed to NH x and quaternary nitrogen (Q), i.e. ascribed reversely. Already in 1987 Bartle, Perry and, Wallace,46 while studying model compounds, determined the following sequence for different forms of nitrogen, additionally distinguishing amino nitrogen: -dibenzo[a,i]carbazole and carbazole), Q -401.4 eV quaternary nitrogen.

Fig. 3

 3 Fig. 3 XPS survey spectrum of carbon nitride (D-600) (a) and spectra of C 1s (b), N 1s (c), and O 1s (d). Peaks labelled as Sat. at 293.8 and 404.3 eV are attributed to the shake-up satellites. Spectroscopic data were processed by the CasaXPS software using a peakfitting routine with Shirley background and asymmetrical Voigt functions.

Fig. 4

 4 Fig. 4 The content of different forms of nitrogen in various models of carbon nitride: Hn (heptazine network) -the model consisting of heptazine building blocks connected into a network, Mmc (melon multi-chain) -the model based on melon built of many chains, Msc (melon single-chain) -the model based on melon built of one chain. The results of XPS measurements of the composition of carbon nitride synthesized at 500, 550 and 600 o C are also presented in the Figure.  -Py nitrogen (N1),  -NH 2 nitrogen (N2),  -NH nitrogen (N3),  -quaternary nitrogen (N4).

6 |

 6 J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx

Fig. 5

 5 Fig.5 The structure of heptazine-based graphitic carbon nitride, a) polyheptazine (fully-condensed), and b) poly(aminoimino)heptazine (melon-type). Carbon, nitrogen and hydrogen atoms are represented as black, blue and red spheres, respectively.

Fig. 6

 6 Fig. 6 XRD patterns of D-600 carbon nitride obtained experimentally (black line) and theoretically calculated fully condensed hexagonal P6m2 cell (blue) and orthorhombic melon-type P21212 cell (red). The upper part of the figure is an enlargement of the bottom diffraction pattern.

Fig. 7 X

 7 Fig. 7 X-ray powder diffraction patterns of gCN-C 3 N 4 synthesized at different temperatures; a) enlargement of the range 6-25 o 2; b) enlargement of the range 24-30 o 2.

Table 2

 2 N 1s, C 1s and O 1s core level binding energies for graphitic carbon nitride condensed at different temperatures.

				500 o C				550 o C				600 o C
			B.E., eV	FWHM % Area At.%	B.E., eV	FWHM % Area	At.%	B.E., eV	FWHM % Area	At.%
	N 1s	N1	398.7	1.17	57.5	23.44	398.7	1.0	62.5	31.30	398.7	1.0	64.8	32.67
		N2	399.6	1.17	22.2	9.04	399.5	1.0	17.1	8.55	399.4	1.0	15.2	7.64
		N3	400.5	1.17	12.1	4.93	400.4	1.0	11.3	5.68	400.3	1.0	11.1	5.58
		N4	401.4	1.17	8.2	3.34	401.4	1.0	9.1	4.57	401.3	1.0	9.0	4.56
			Total N:			40.75	Total N:			50.09	Total N:			50.45
	C 1s		284.6	1.4	43.1	24.19	284.6	1.1	22.3	10.76	284.6	1.1	20.9	10.00
			-	-	-	0.00	287.0	1.1	0.8	0.38	286.9	1.0	1.4	0.69
			288.0	1.2	56.9	31.96	288.2	1.1	76.9	37.19	288.2	1.0	77.7	37.2
			Total C (in C-N):		31.96	Total C (in C-N):		37.57	Total C (in C-N):	37.89
	O 1s		533.1	2.7	100.0	3.10	532.8	2.5	100.0	1.58	532.6	2.5	100.0	1.67

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Table 3

 3 Carbon nitride structures, equations for calculating the number of nitrogen atoms (N -number of heptazine subunits, n -number of chains) and the percentage of different forms of nitrogen.

Table 4

 4 Unit cell and structural parameters of gCN synthesized at different temperatures (space group P2 1 2 1 2, No. 18). The measurement error is given in parentheses. The structure models used for the refinement of gCN were placed in the ESI.

	Temperature [ o C]	500	550	600
	(002) [2]	27.380	27.508	27.723
	(002) d [Å]	3.254	3.234	3.215
	a [Å]	16.352(6)	16.264(6)	16.351(6)
	b [Å]	12.497(4)	12.386(4)	12.309(4)
	c [Å]	6.5018(4)	6.4682(5)	6.4379(4)
	V [Å 3 ]	1328.67(58)	1303.01(56)	1307.92(26)
	size (0 0 2) [nm]	7	8	10
	number of layers	22	25	31
	(210) [2]	12.900	12.945	13.014
	(210) d [Å]	6.857	6.833	6.797

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 7

Acknowledgements

The work was supported by grant no. POWR.03.02.00-00-I023/17 co-financed by the European Union through the European Social Fund under the Operational Program Knowledge Education Development.

ARTICLE

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Conflicts of interest

There are no conflicts to declare.


Dr Quentin Lenne

Alice Mattiuzzi

Ivan Jabin

Nicolas Le Poul

Dr Yann R Leroux
email: yann.leroux@univ-rennes1.fr

Corinne Lagrost
email: corinne.lagrost@univ-rennes1.fr

Dr A Matiuzzi

Prof. I Jabin

Dr N Le

Poul Cemca

Functionalizing Gold Nanoparticles with Calix[4]arenes Monolayers for Enhancing Selectivity and Stability in ORR Electrocatalysis

Keywords: surface functionalization, electrocatalysis, gold nanoparticle, diazonium, oxygen reduction reaction

The deliberate surface modification of nanocatalysts with organic ligands has recently emerged as a promising strategy to boost their efficiency, durability and/or selectivity in key electrocatalytic processes. The interface between the metallic interface and the immobilized ligands promotes high electrocatalytic activity. Herein, the oxygen reduction reaction activity of gold nanoparticles functionalized with a covalently-bound monolayer of calix[4]arenes is compared with commercially available gold nanoparticles, classically stabilized through electrostatic adsorption of citrates onto the gold surfaces. Nanocatalysts coated by calix[4]arenes show enhanced selectivity and stability compared to their citratestabilized counterparts. These nanohybrids exhibit excellent activity with a dominant 4-electron reduction of O2 with good electrocatalytic performances along with a high robustness under operation as revealed by X-ray photoelectrons spectroscopy analyses.

Introduction

Organic ligands or capping agents are traditionally employed for preparing metallic nanoparticles (NPs) with controlled shape and size in wet chemistry. [1][2][3][4] Their roles are to prevent the aggregation of nanoparticles in solution and to control their growth at the crystallographic level. [4] For applications in electrocatalysis, the ligands are considered as poisonous species, blocking an important proportion of their active sites and deactivating the catalysts. The removal of ligands from the nanoparticles surface without altering the specific structure that rules their catalytic function, is a major challenge. [5][6][7]. Interestingly, organic ligands are commonly designed and employed in homogeneous catalysis to steer the activity and selectivity of metal centers. [8,9] The presence of organic ligands allows better control of selectivity. In line with this concept, the chemical surface modification of metallic nanostructures has just emerged as a promising strategy to increase their catalytic performances as recently reviewed. [10] Recent reports have highlighted that organic functionalization of metallic nanoparticles can boost their electrocatalytic properties through local interfacial steric or electronic effects. [11,12] Far from having a detrimental impact, the ligands are found to have a beneficial role. The nature of the ligands, [13] and/or the interfacial bonding, [14] promote high electrocatalytic activity, selectivity and durability. [10] The immobilization of organic molecules to metallic surfaces includes the chemisorption of monomers, polymers or surfactants, the self-assembly, the covalent grafting or the electrostatic adsorption of charged molecules (e.g. citrates, polyelectrolytes). The strength of interaction between surface and ligands depends on the employed procedures. Whereas thiol molecules are known for decades to efficiently bind gold nanoparticles (AuNPs) via Au-S bonds, [15,16] more recently, Au=C (carbene), [17] Au-CC (acetylide), [18] or Au-C (through aryl diazonium salts reduction), [19] provide robust interfacial bonds with strong metal-ligands interactions. The aryldiazonium reduction leads to strong interaction between the metallic surfaces and the aryl moieties, with adsorption energies over 200 kJ.mol -1 . [20] In contrast, electrostatic adsorption, involved for A c c e p t e d m a n u s c r i p t instance in citrate-stabilized gold nanoparticles corresponds to a weaker interaction with binding energy lower than 85 kJ.mol -1 . [21] We have recently developed a unique strategy to prepare dense and compact monolayers on a wide range of materials including nanomaterials thanks to the reductive grafting of calix [4]arene-tetradiazonium salts. [22,23] The calix [4]arene-tetradiazonium cation exhibits a cone-constrained structure made up of four aromatic units linked by methylene bridges, allowing a clean control of the formation of robust monolayers. [24] The large rim is equipped with four reactive diazonium functions, hence providing multiple anchoring points. [24] Such a multidentate structure enhances the stability and the organization of the monolayer, similar to what was described in the literature with other multidentate anchoring systems. [25] Importantly, we demonstrate that gold nanoparticles functionalized with calix [4]arene-tetradiazonium salts have exceptional colloidal robustness toward halides or extreme pH. [26] Such a property is potentially very appealing for applications in electrocatalysis, notably for oxygen reduction reaction (ORR) since the experimental conditions are demanding. ORR is one of the most important electrochemical reaction in energy conversion technologies, e.g. electrocatalytic water splitting, [27,28] metal-air batteries [29,30] and fuel cells. [31,32] The cathodic oxygen reduction reaction is a slow multielectronic reaction which therefore occurs at high overpotential in the absence of a suitable catalyst. The 4-electron reduction process is generally desired and produces water. Platinum and platinum-based alloys are still the best ORR catalysts for proton exchange membrane fuel cells (PEMFCs) operating under acidic conditions [5,6,[33][34][35] although, breakthroughs have been recently achieved with non-noble metals electrocatalysts. [30,36] Alkaline fuel cells could represent an alternative technology for which gold, silver and metal oxides are even better catalysts than Pt. [37][38][39][40][41] Moreover, anions, which are possible poisonous species, undergo lower adsorption energies in alkaline media than in acid ones. [42] In this context, AuNPs efficiently catalyze ORR, mainly through an indirect 4e-(2+2 e-) pathway. [37,38,41] A c c e p t e d m a n u s c r i p t Herein, we present the electrocatalytic activity toward ORR of AuNPs functionalized with a covalently-bound monolayer of calix [4]arenes (i.e. Calix-AuNPs). The monolayer allows wellorganized structure with a fine molecular control of distribution of ligands. So far, the effect of a well-organized and robust structure on the ORR properties of the modified AuNPs have not yet been explored. Especially, the fine control of ligands distribution on AuNPs surfaces remains highly challenging. In addition, the rigid structure of calix [4]arenes moieties allows a fine spatial control of the small rim substituents which is imposed by the small rim geometry.

Thus the acetic acid arms are perfectly positioned at the solid/liquid interface, able to modulate the interaction between electrolyte and the catalytic surface. The electrocatalytic performance of the calix-AuNPs is compared to AuNPs modified with citrates (i.e., Citrate-AuNPs) (Figure 1). Recent works have indeed reported the good activity of citrates-capped AuNPs of 15 nm size as electrocatatalysts for ORR. [13,43] Citrates are the most classical ligands used for commercially available AuNPs. They stabilize AuNPs through electrostatic adsorption by forming multilayers in a disordered arrangement. [44] The interaction between metallic catalysts and ligands in citrate-AuNPs is then basically different from that involved in calix-AuNPs, making the comparison of the two materials attractive. The results are discussed in terms of nanohybrids efficiency and stability as well as mechanistic pathway for ORR.

Results and Discussion

Gold nanoparticles functionalized by calixarenes through diazonium reduction were synthesized as previously described. [26] Briefly, a solution of HAuCl4 is reduced in the presence of calix [4]arene-tetraacetic acid-tetradiazonium salts via the addition of NaBH4 to yield calix [4]arene-functionalized AuNPs (i.e. calix-AuNPs) of 6.1 ± 1.9 nm. Upon addition of the reducing agent in the solution, aurate salts and aryl diazoniums are transformed to gold clusters and aryl radicals, respectively. The aryl radicals are able to react with the gold surface forming a strong bond between the calix [4]arenes molecules and the gold nanoparticles.

Because of the constrained macrocyclic structure of the calix [4]arene, the nanoparticles are covered with a monolayer of calixarenes as previously demonstrated. [24,26] Growth of nanoparticles is presumably stopped by the grafting of calix [4]arene-based radicals. Figure 2 shows the particle size distribution after a simple centrifugation step. A good size dispersity is obtained for the functionalized calix-AuNPs with a primary size of 6.1 nm. Taking into account that the size of the AuNPs has a drastic impact on their electrocatalytic activity that increases as nanoparticle size decreases, [43,45] it is necessary to work with citrate-AuNPs and calix-AuNPs with close size. Consequently, citrate-AuNPs of 5 ± 1 nm will be used in the following.

A c c e p t e d m a n u s c r i p t

The ORR electrocatalytic activities of the two surface-modified AuNPs have been measured using a platinum ring (Pt)/glassy carbon disk (GC) rotating ring-disk electrode (RRDE) in O2saturated aqueous 0.1 M KOH, after deposition of AuNPs onto the GC disk surface (Figure 3).

In alkaline solutions, oxygen (O2) can be reduced to hydroxide anions (OH -) by a direct 4electron pathway: [46] O2 + 2 H2O + 4 e - 4 OH -; E° = 0.401 V vs. SHE or it can follow an indirect 2 + 2 electrons pathway where O2 is first reduced to hydroperoxide ion: [46] O2+ H2O + 2 e - HO2-+ OH -; E° = -0.076 V vs. SHE which is then further reduced to hydroxide anions in a second step: HO2 -+ H2O + 2 e - 3 OH -; E° = 0.878 V vs. SHE or by disproportionation:

2 HO2 - 2 OH -+ O2
The ORR onset potential of calix-and citrate-AuNPs are both close to 0.90 V vs. RHE, corresponding to 330 mV and 270 mV overpotential values, respectively (Figure S4, Supporting Information). Half-wave potentials were determined as 0.80 and 0.83 V vs. RHE, respectively. Such values are comparable to those displayed by commercial Pt/C catalysts, [47][48][49] and other metallic electrocatalysts [50,51] and are even significantly more positive than other AuNPsbased catalysts (Table S2, Supporting Information). [52,53] In addition, this observation does show efficient electrocatalytic activity in ORR despite the capping of the metal cores, especially in the case of a robust capping as with the covalently-bound calix [4]arenes onto AuNPs surface.

The amount of hydroperoxide ions (HO2  ) generated during ORR at the GC disk was monitored by collection experiments at the Pt ring electrode which was set to a potential of +1.70 V (vs. RHE) to promote hydroperoxide oxidation (Figure 3, top curves). It is clear from Figure 3a that From RRDE experiments, the number of electrons transferred per O2 molecule (n) and the proportion of HO2  anions produced (% HO2 -) can be determined. Figure 4 displays the variation of n and % HO2 -as a function of the potential during ORR (see Experimental Section for details). Very different behaviors are observed for calix-and citrate-AuNPs. With citrate-AuNPs, the process exhibits a variation of n values from 3.25 to 3.8 over the potential range (+0.8 V to -0.4 V vs RHE) (Figure 4a). The variation of n is correlated to the production of HO2  whose proportion varies between 5 to 30 % (Figure 4b). At high overpotentials, n decreases to 3.25 and this value corresponds to the maximum of HO2  production (at +0.4 V vs. RHE), in agreement with the shape of the LSV curves displayed in Figure 3b (bottom). This indicates that HO2  is formed as intermediate reduction product and is further reduced at more negative potentials, reaching an overall 4e-ORR process below -0.2 V vs. RHE (Figure 4).

Hence, the reduction of O2 at citrate-AuNPs proceeds through a stepwise 2e-+ 2e-pathway. In sharp contrast, calix-AuNPs show a constant n value of 3.9 over the entire potential range along with a very low level (< 5%) of HO2  detected (Figure 4b, red curve). Such a result is very A c c e p t e d m a n u s c r i p t interesting and quite unique for Au electrocatalysts. [13,43,[52][53][54][55][56][57] Worth mentioning is that a stepwise 2e-+2e-process is generally observed with gold nanocatalysts in alkaline media as seen with citrate-AuNPs. Two-wave polarisation curves are generally obtained where HO2 -is formed at the potential at the first wave, then can further undergo a complete reduction to OH - at more negative potentials thanks to the good disproportionation activity of gold nanomaterials. [41,52] Bulk gold displays poor HO2 -disproportionation activity and then predominantly reduce O2 with 2 electrons. Only some Au single crystals Au(100) show exceptional catalytic 4e -activity with almost no HO2 -detection at low overpotentials but in a limited potential range. [58] Interestingly, carbon nanotubes-amphiphilic nitrilotriacetic-dyine lipid-gold nanohybrids catalysts were recently found to allow a predominant 4e-ORR process and low production of HO2 -(< 10 %) in alkaline solution but in a limited potential range, close to the onset potential. The kinetic current density (JK) corresponds to the intrinsic activity of the catalysts and allows a quantification of their performance. JK is further evaluated by using the Koutecký-Levich equation in the RDE voltammetric experiments to correct from mass-transfer contribution:

1 𝐽 = 1 𝐽 K + 1 𝐽 L = 1 𝐽 K + 𝜔 -1/2
𝐵 (Equation 1)

𝐵 = 0.62 𝑛𝐹𝐶 O 2 𝐷 O 2 2 3 ⁄ 𝜈 -1 6 ⁄ (Equation 2)
where JL is the diffusion-limiting current density, n the overall number of electrons transferred during ORR, F the Faraday constant (96 485 C.mol -1 ), CO2 the saturated O2 concentration in the electrolyte (1.2 10 -3 mol.L -1 ), [56] DO2 the diffusion coefficient of O2 in the electrolyte (1.9 10 - 5 cm².s -1 ), [56] 𝜈 the kinematic viscosity of the solution (0.01 cm².s-1) and ω is the angular frequency (in rad.s -1 ). Both series of AuNPs show good linearity of the Koutecký-Levich plots which demonstrates first-order reaction kinetics toward dissolved oxygen (Figure S5, Supporting Information). At +0.75 V vs. RHE, the normalized kinetic current density (per electrochemically active surface area, see Supporting Information), JK,sp was found equal to A c c e p t e d m a n u s c r i p t 1.22 and 2.73 mA.cm -2 for calix-and citrate-AuNPs, respectively. These values demonstrate good to very good performances of the two series of AuNPs compared to already reported specific activities of various metallic electrocatalysts including bare Pt and Au nanomaterials (see Table S2).

For both series of AuNPs, the mass-transfer corrected Tafel plots display two linear segments of different slopes (Figure 5). Such a change of the slopes is classically reported for ORR processes in both alkaline and acidic media (Table S2, Supporting Information). At low current density, we found absolute slopes equal to 42 mV/decade and 54 mV/decade for calix-and citrate-AuNPs, respectively. In the higher current density region, the corresponding absolute Tafel slopes were 124 mV/decade and 104 mV/decade, respectively (Figure 5). These values are rather close to the typical slopes of 60 mV/decade (low J) and 120 mV/decade (high J) generally found for ORR processes. It was shown that the former could correspond to a pseudo two-electrons transfer mechanism possibly involving the breaking of O-O bonds as the rate determining step. [60] At higher J, the rate-determining step was considered to be the first electron transfer. [61] One should note that microkinetic analyses are currently discussed and some authors have proposed that the change of the Tafel slope is not related to a change of ratedetermining step but more likely due to the adsorption of species that could alter the kinetics. [62] A c c e p t e d m a n u s c r i p t The two AuNPs hybrid systems show good stability upon long term chronoamperometry measurements at 0.75 V vs. RHE for 12 h in aqueous O2-saturated 0.1 M KOH solution (Figure S6, Supporting Information). The calix-AuNPs have an even better stability than the citrate ones. Classically, the durability of ORR catalysts is probed through comparison of LSV curves before and after accelerated stability tests. They generally consist of one thousand cyclic voltammograms at 500 mV.s -1 by sweeping the potentials between 1.2 V and -0.35 V (vs. RHE) in O2-saturated 0.1 M KOH solution. LSV curves were recorded after the accelerated stability tests and compared to the original ones (Figure 6). In line with the long term chronoamperometry measurements, the durability of calix-AuNPs and citrate-AuNPs are good, with a clear superior stability of calix-AuNPs compared to citrate-AuNPs. Almost no variation of the LSV curve is detected after the cycling process for calixarene nanohybrid catalyst. We performed a further study of the stability of the hybrid catalysts through measurements of electrochemically active surface area (ECSA) and X-ray photoelectron (XPS) spectra before and after accelerated stability tests. ECSA were estimated from cyclic voltammetries in acidic medium purged from O2 through the integration of reduction peak of gold oxides (Figure S7, Supporting Information). While only 22% of citrate-AuNPs remains active after stability tests, 63% of calix-AuNPs is still operative. XPS analyses were performed before and after stability tests to evidence any possible change of the chemical composition after ORR experiments (Figure 7). High-resolution core level spectra in the C 1s region are peak-fitted and display several components (Figure 7). For Calix-AuNPs, a main peak centered at 284.5 eV could be mostly attributed to aliphatic and aromatic carbons of calix [4]arenes as well as possible contribution of the substrate itself (Figure 7a). Component at 286.0 ± 0. S1, Supporting Information). Such a value is in good agreement with the 1:1 ratio expected from the molecular structure of calix [4]arene. This observation indicates that there is no degradation of the hybrid calix-AuNPs catalysts during ORR. to the chemical structure of the citrate ligands stabilizing the AuNPs as previously described. [44,64] The component ratio COOH/COH is found equal to 3 as expected from the molecular structure of citrate (Table S1, Supporting Information). After stability tests, a significant shift of the peak corresponding to C-C/C-H is observed along with a clear decrease of the C-OH and COO-components. The component ratio COOH/COH decreases to 1.7 (Table S1, Supporting Information). Thus, the XPS results suggest a chemical decomposition of the citrate molecules stabilizing the AuNPs. The loss of activity for the citrate-AuNPs is directly related to this chemical degradation. Upon degradation of their citrate shell, AuNPs are much prone to aggregation, explaining the dramatic decrease in ECSA measurements after stability test. It may be inferred from these observations that the active catalytic surface for citrate-AuNPs is actually the gold surface being rid of citrates molecules. Contrariwise, the monolayer of calix [4]arenes remains intact, suggesting that in case of calix-AuNPs, the whole nanohybrid system is active.

A c c e p t e d m a n u s c r i p t

Conclusion

In summary, chemically modified gold nanoparticles (5 and 6 nm) with citrates and calix [4]arene-based organic ligands were shown to be efficient electrocatalysts for ORR in alkaline media. Electrochemical analyses clearly demonstrate that the calix [4]arenefunctionalized AuNPs outperform on several points the citrate-AuNPs, despite a slightly larger size. This is ascribed to interfacial steric and electronic effect induced by the covalent functionalization of the AuNPs with monolayers of calix [4]arenes. The performances of the calix-AuNPs hybrid system are particularly interesting with a rather low overpotential and a predominant 4 e-ORR process, equaling Pt-based materials under similar conditions. [65] The stability of AuNPs under operation is significantly enhanced when they are functionalized with calix [4]arenes platform compared to citrates capping. The calix-AuNPs nanohybrids compare favorably with other reported AuNPs electrocatalysts (see also Table S2, Supporting A c c e p t e d m a n u s c r i p t Information). [14,43,[53][54][55][56][57] Such a robust electrocatalyst appears promising for application in fuel cells or O2 sensors, and opens perspectives to expand the calixarene functionalization strategy toward other electrocatalytic reactions. Further works are currently under progress in order to tune the electrolyte/catalytic metal interface by modifying the small rim functions.

Experimental section

Chemicals. Solvents and reagents for the syntheses were at least of reagent grade quality and were used without further purification. Citrate capped 5 nm AuNPs were purchased from Ted Pella, Inc (manufactured by BBI solutions, https://bbisolutions.com/eu/gold-nanoparticles-5nm.html).

Synthesis and characterization of functionalized nanoparticles. Calix [4]arene functionalized gold nanoparticles were synthesized according to an one-pot procedure. Gold salt HAuCl4.3H2O purchased from Sigma Aldrich (>99.9%, 18 mg, 0.0457 mmol) was dissolved in dry acetonitrile (20 mL) and mixed with an acetonitrile solution (20 mL) of calix [4]arenetetraacetic acid-tetradiazonium salts (Figure S2, Supporting Information). The calix [4]arenetetraacetic acid-tetradiazonium salts were synthesized as previously reported. [24,26] [Caution! Although we have not encountered any problem, it is noted that diazonium salts derivatives are potentially explosive and should be handled with appropriate precautions.] The reaction mixture was stirred vigorously at 0°C under Ar and an aqueous solution of NaBH4 (Sigma Aldrich, ≥98%, 0.5 mg in 1 mL) was added dropwise. The color of the reaction mixture changed from yellow to dark ruby. After 2 hours of vigorous stirring at room temperature, the reaction mixture was centrifuged at 10 000 rpm for 20 min, resulting in gold nanoparticles functionalized with calix [4]arenes ligands, calix-AuNPs (18 mg, 0.0236 mmol). Calix-AuNPs were washed by resuspension/precipitation in 1 M NaOH and 1 M HCl aqueous solution respectively. Finally, after removal of supernatant, milli-Q water (18.2 M.cm) was added to Electrochemical analyses. The ORR activity was evaluated using a rotating Pt/GC ring-disk electrode (37% collection efficiency) controlled by a MSR Rotator from Pine Research. The GC electrode preparation (deposit of catalysts) is fully described in the Supporting Information.

The LSV scans were recorded using an Autolab PGSTAT302N potentiostat-galvanostat (Metrohm) equipped with a BIPOT module, in a conventional three-electrode electrochemical cell at a scan rate of 10 mV.s -1 in a 0.1 M KOH solution saturated with O2. A graphite rod and a Hg/HgO, NaOH (1 M) electrode were used as counter and reference electrodes, respectively.

Voltammograms were recorded with the Metrohm Nova 1.11 software. The potentials were not corrected from iR-drop. The potentials are reported against the reversible hydrogen electrode according to E (RHE) = E (Hg/HgO) + 0.059 pH + 0.140. The preparation of the GC electrodes and its characterization (SEM) is fully described in SI.

The number of electrons exchanged can be quantified using the disk and ring currents according where ID is the modulus of the disk current, IR is the ring current corresponding to the oxidation of HO2 -and N is the collection efficiency. N was determined as 0.37 using the ferro/ferricyanide couple, in agreement with manufacturer indication. [66] The fraction of HO2 -that is produced in the reaction can be calculated from: % HO2 -=100 where Qoxides is the charge integrated from the voltammetric peak corresponding to the reduction of gold oxides, m is the mass of gold deposited and C is the monolayer adsorption charge on Au (400 µC cm -2 ). [67] ECSA was found to be 0.36 cm 2 (0.11 cm 2 µg -1 Au) and 1.52 cm 2 (0.092 cm 2 µg -1 Au) for citrate-and calix-AuNPs, respectively.

X-ray photoelectron spectroscopy measurements. XPS data have been collected by a Kratos Axis Nova spectrometer using the Al Kα X-ray source working at 1486.6 eV and using a spot size of 0.7 x 0.3 mm². Survey spectra (0-1000 eV) (Figure S7) were acquired with an analyzer pass energy of 160 eV (0.5 eV/step); high resolution spectra used a pass energy of 40 eV (0.1 eV/step). Binding energies were referenced to C1s peak at 285 eV or to Au 4f7/2 at 84 eV. The core level spectra were peak-fitted using the CasaXPS Software, Ltd. Version 2.3.18. U2

Tougaard was used as background. The peaks areas were normalized by the manufacturersupplied sensitivity factor (SC1s = 0.278, SF1s = 1, SO1s = 0.78, SS2p = 0.668, SAu4f = 6.25). Since the XPS measurements cannot be made with the RRDE electrodes due to their height, deposit For each sample, XPS spectra were recorded on three different locations, and we did not observe any difference.

Figure 1 .

 1 Figure 1. Schematic representation of (a) AuNPs functionalized with calix[4]arenes (calix-AuNPs) and (b) AuNPs stabilized with citrates (citrate-AuNPs).

Figure 2 .

 2 Figure 2. a) TEM micrographs and b) corresponding particle core size histogram of calix-AuNPs.

A

  c c e p t e d m a n u s c r i p t almost no hydroperoxide ions are produced during ORR with calix-AuNPs whereas it can be clearly detected for citrate-AuNPs (Figure3b).

Figure 3 .

 3 Figure 3. RRDE voltammetric curves in O2-saturated 0.1 M KOH of GC disk electrodes modified with either (a) calix-AuNPs or (b) commercial citrate-AuNPs (bottom curves). Ring electrode (top curves): Pt. v = 10 mV.s -1 , 200 rpm <  < 2000 rpm, Ering = 1.70 V vs RHE.

  [59] However, a very efficient disproportionation activity of this nanohybrid material was evidenced in the whole potential range [1; 0 V vs RHE].[59] Because the calix-AuNPs display « apparent» direct 4-e-process on the whole potential range with a very low HO2 -formation (< 5%) and almost no variation of the number of electron exchanged (4), they exhibit enhanced selectivity towards the 4 e-process, close to what it is obtained with Pt electrocatalysts in acidic media. either through a direct 4e-process or a superior HO2  disproportionation.A c c e p t e d m a n u s c r i p t

Figure 4 .

 4 Figure 4. Potential dependence of (a) the number of electrons, n, and (b) the proportion of HO2  (% HO2 -) produced during ORR experiments for calix-AuNPs (red lines) and citrate-AuNPs (black lines).

Figure 5 .

 5 Figure 5. Mass-transfer corrected Tafel plots for O2 reduction for (a) calix-AuNPs and (b) citrate-AuNPs in O2-saturated aqueous 0.1 M KOH. v = 10 mV.s -1 ; ω = 1600 rpm.

Figure 6 .

 6 Figure 6. ORR polarization curves for calix-AuNPs (red curves) citrate-AuNPs (black curves) deposited onto the GC disk and recorded at room temperature in O2-saturated aqueous 0.1 M KOH before (dotted curves) and after (solid line curves) accelerated stability tests. v= 10 mV s -1 .  = 1600 rpm.

  3 eV corresponds to the ether bonds (C-O-C) of the calix[4]arene core while the component at 289.1 ± 0.4 eV eV (O-C=O) is assigned to the carboxylic acid groups substituting the small rim of the calix[4]arene core. Two other components at 291.5 ± 0.2 eV (CF2) and 293.2 ± 0.2 eV (CF3) are due to the Nafion used in these experiments. Except a small erosion of Nafion coverage, the peak-fitted C A c c e p t e d m a n u s c r i p t 1s signal is fully comparable before and after ORR stability tests (Figure 7a,b). Especially, the component ratio COC/COOH is close to 1, i.e. 1.03 and 1.2 before and after ORR experiments, respectively (Table

Figure 7 .

 7 Figure 7. High-resolution XPS core level spectra of the C 1s region before a), c) and after b), d) ORR stability experiments on a), b) deposit of calix-AuNPs and c), d) citrate-AuNPs. The small contribution at 284.3 eV characteristic of C sp² carbon (c and d) is attributed to the carbon substrate.[63]

A

  c c e p t e d m a n u s c r i p t dilute ionic species. Samples were concentrated by centrifugation to obtain a 2 mgAuNPs/mL concentration. Transmission Electron Microscopy. Morphology and size distribution of the samples were investigated by high-resolution transmission electron microscopy (HRTEM), using a Jeol 2100 with an acceleration voltage of 200 kV on which the images were recorded on a GATAN Orius 200 CCD camera. The size distribution was estimated using ImageJ software, version 1.52a. Deposition homogeneity was inspected by scanning electron microscopy (SEM) on a Jeol JSM 7100 combined with an energy dispersive X-ray spectroscopy Oxford Instruments AZtecEnergy detector.

)

  The onset potential Eonset was determined on the LSV curves as the potential corresponding to 1/100 of IL, IL being the diffusion limiting current.Surface of electrochemically active AuNPs (ECSA) was measured by cyclic voltammetry at 100 mV.s -1 in an aqueous 0.1 M HClO4 purged with Ar. A graphite rod and a SCE electrode, KCl (sat.) were used as the counter and reference electrode, respectively. Potential was swept until stabilization of the signal.

A c c e p t e d m a n u s c r i p tof

  AuNPs were performed onto screen-printed carbon electrode Dropsens®. The modified electrodes subsequently underwent ORR stability experiments (1000 cycles at 500mV.s -1 from 1.20V to -0.35V vs RHE) under the same experimental conditions used for ORR measurements with RRDE. Due to their good mechanical stability onto the carbon electrode, the deposit of citrate-AuNPs were not covered with Nafion, to avoid the screening effect of the Nafion film.

Acknowledgements Q.L. thanks the French Ministry of Research for financial support. J. Hamon (Institut des Matériaux de Nantes, Nantes, France) is warmly thanked for his help in recording the X-ray photoelectrons spectra. The authors are grateful to L. Rault for the assistance in TEM experiments performed on THEMIS platform (CPER-FEDER 2007-2014) and to F. Gouttefangeas for SEM experiments performed on CMEBA platform (THEMIS/CMEBA, ScanMAT, UMS 2R011 University of Rennes 1-CNRS, Rennes, France).

Conflict of Interest I. J. and C. L. are shareholders of X4C. I.J. is a consultant for X4C. All other authors declare that they have no conflict of interest.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

TOC graphic

Designing a gold-calix [4]arene nanohybrids for enhancing selectivity and stability in ORR.

A c c e p t e d m a n u s c r i p t


Vincent Schoepff

Istvan Almasi

Khalil Amgarou

Frank Becker

Frédérick Carrel

Federico Carvajal

David C Gaboriau

Mehdi Gmar

Andras Kelemen

Andras Kovacs

Becker

István Almási

András Kelemen

András Kovács

Éva Kovács-Széles

Hermine Lemaire

Nabil Menaa

Dávid Mesterházy

Luc Morat

Ciaran G Morrison

Israel Perez-Llopis

Wolfgang Raskob

Marie-France Robbe

Sándor Szabo

Isabelle Testard

Dmytro Trybushnyi

Nicolas Ugolin

Muriel Viau

Árpád Vincze

BOOSTER: Development of a toolbox for triage of large groups of individuals exposed to radioactive material

Keywords: Decision making, Decision support systems, Dosimetry, Environmental monitoring, Gamma-ray cameras, Radiation instrumentation, Radiation monitoring, Safeguards applications, Security applications, Radioactive pollution, Radiation imaging systems, Miniature radiation cameras, Gamma-ray spectroscopy, Gamma-ray detectors, Dose rate

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.



Abstract -The effective management of an event involving the exposure of a large number of people to radioactive material requires a mechanism for fast triage of exposed people. BOOSTER is a project founded by the European Union under the Seventh Framework Programme, addressing this requirement. It is a capability project designed to provide an integrated system which could easily be deployed and used. For this purpose, the BOOSTER consortium, relying on the expertise of seven members, researches and develops new approaches to allow an effective and fast management of most kind of nuclear threats. BOOSTER System was designed to help first responders mitigate the crisis by providing the necessary information to quickly assess the radiological situation, to support triage staff in performing an efficient and fast categorization of the potentially affected victims, and to give medical staff crucial information for further treatment at medium or long term post-accident. Index Terms -Biodosimetry, Crisis management, Nuclear accident, Radiological attack, Triage, Radioactive pollution.

I. INTRODUCTION

ecent nuclear accidents, such as the Fukushima event, demonstrated people's fear regarding nuclear risks. Moreover, 9/11 attacks showed the reality and impact of terrorist threat which forced societies to quickly adapt and develop new technological solutions. Large scale terrorist attacks or accidental scenarios, involving a radiological risk, could have dramatic and long-lasting consequences from a sanitary but also psychological point of view.

The effective management of an event involving the exposure of a large number of people to radioactive material, either accidental or following a malevolent use of nuclear material requires a mechanism for fast triage of exposed people. BOOSTER is a project founded by the European Union under the Seventh Framework Programme, designed to provide an integrated system which could easily be deployed. For this purpose, the BOOSTER consortium developed new approaches to allow an effective and fast management of most kind of nuclear threats. Based on an identified scenario of terrorist attack, reflecting the threats likely to be faced, BOOSTER System was designed to help first responders mitigate the crisis by providing the necessary information to quickly assess the radiological situation, to support triage staff in performing an efficient and fast categorization of the potentially affected victims, and to give medical staff crucial information for further treatment at medium or long term postaccident.

In this paper, an overall description of BOOSTER System functionalities for optimizing crisis management following an accidental or malevolent nuclear event is presented. An emphasis is made on the first results obtained for the adaptation of the GAMPIX gamma camera during laboratory tests. In parallel, summary results of other developments and adaptations is presented allowing a first assessment of BOOSTER performances.

II. ARCHITECTURE OF THE SYSTEM

A. A system based on current procedures

The BOOSTER System architecture was imagined to fit current procedures for radiological crisis management. Once any radiological contamination is detected on the site of an event, the BOOSTER System is deployed by the first responders. In addition, the necessary command & control echelons are established according to the crisis magnitude.

BOOSTER: Development of a Toolbox for Triage of Large Group of Individuals Exposed to Radioactive Material

In the case of the management of a crisis (accident, terrorist attack…), involving nuclear or radiological material, different areas are commonly defined around the scene [START_REF]Manual for First Responders to a Radiological Emergency[END_REF]. These areas are established in order to exclude and guard the dangerous zone for unauthorized people, protect and control the public, and minimize the spread of contamination in surroundings.

Exclusion area and Controlled area

The Exclusion area is the area directly affected by the event. It is subject to high levels of radioactivity and contamination. In this area only duly protected first responders perform the rescue and manage the evacuation of the victims. These first responders (mainly firemen) deploy several measurement devices belonging to the BOOSTER System in order to gather crucial information that helps for situation assessment and coming triage. With these components of the BOOSTER System, first responders will provide data on radioactivity levels, taken from deployed sensors, get an overview of the situation, establish the Exclusion area and Controlled area perimeters and assess by calculation the dose received by victims by external irradiation, depending on their position at the moment of the incident.

The Controlled area is a secured area surrounding the Exclusion area. Its perimeter is defined according to the dose rate levels (i.e. an area where the dose rate is lower than 0.3 µSv/h according to IAEA guidelines) [START_REF]Manual for First Responders to a Radiological Emergency[END_REF]. The Controlled area is used as a fallback position for the first responders deployed in the Exclusion area and a safer interface for victims evacuation. The first phase of the triage, which consists in a medical triage determining the level of injuries encountered by victims, is carried out in the Controlled area.

Decontamination area

Before going out of the Controlled area, and in order to minimize the spread of contamination, victims and involved people (members of emergency services, for instance) are taken to a contamination control checkpoint. If a victim is not contaminated, this person can continue the triage circuit. On the contrary, a decontamination process is applied in the Decontamination area, where several measurement devices are deployed to assess the level and the position of external contamination on individuals.

Support area

Following the decontamination (if required), a deeper analysis of victims for radiological triage is performed once the affected persons arrive to the Support area. This way, the victim triage is accelerated and each person receives adequate treatment according to their radiological status. Those analyses rely on newly developed biodosimetric tools and tests on either biological samples or belongings originating from victims.

Remote Command and Control area

This area is located away from the event area; nevertheless several command and control components which compose the first echelons are also located in the other usage areas. All information from the Controlled area as well as second level global information from the Support area is received in the Command and Control area and assists the authorities and higher level crisis managers in mitigating the incident effects and manage the available resources in an optimum way.

B. Overview of BOOSTER system components

The BOOSTER system was designed to encompass three main objectives: the evaluation of environment radiological situation, the quick on-site first triage of victims and the collection of more precise estimation of medical impact for medium/long term follow-up.

In order to achieve these goals, a set of tools was chosen and designed to provide the necessary information. Table I summarizes the integrated components, their goals and main characteristics. These tools, which will be described in this article, mainly consist in: deployable sensors for situation assessment and first responders information (presented in section III), fast-response on-site methods for the evaluation of dose received by victims in order to perform a first triage in case of mass casualties (presented in section IV), and longerterm in-laboratory techniques for more precise determination of radiological exposure for refined medical treatment (presented in section V).

C. Compactness, ease-of-use, quickness and scalability

Due to the particularly difficult work conditions in the Exclusion and Controlled Areas, as explained in section II-A, only duly protected first responders are allowed to operate in this area, the focus is made on the compactness, portability and ease-of-use of the deployed sensors. This portable toolbox allows emergency services to speed up and optimize the first phase of the triage without having to focus their effort toward measurement devices. Therefore, all components were developed or modified in order to be autonomous and easily wearable, or at least easily deployable, by first responders.

In order to speed up the triage process, all in-field analyses were designed to give their results in less than twenty minutes and allow for parallel analyses. As operators carrying out biological samplings (hair follicles, blood, buccal cells…) required for analyses are not necessarily part of the medical staff, sampling material was chosen and especially adapted to be non-invasive. The software architecture is also adapted for use by non-experts. For instance, most sensors are automated for getting the results and feedback information to upper echelons of command and control by themselves.

As no event is comparable to another, the BOOSTER system was designed to be scalable, i.e. the composition of the system can be modified to fit the magnitude of the event. The toolbox was scaled for a given number of affected people. However, it is possible to multiply the toolboxes deployed on the site of the event and to connect all of them to the same Command & Control and DSS systems to fit the range of casualties.

D. Coordinated network and interoperability

Sensor platforms, including a GPS device were developed to be deployed in the affected area. These platforms were adapted to receive each sensor and have the availability to send the gathered information to a coordinated network based on the SIMACOP system developed by UPV [START_REF] Esteve | SIMACOP: Small Units Management C4ISR System[END_REF]. The core software processes the information from sensors and interpolates to get a picture of the radiological situation in the area of interest. It also provides an evaluation of the exposure and contamination status of people, based on the data from deployed sensors and victims' position in the area.

Information from sensors, estimation from software and data from external sources are processed in a decision support system. This decision aiding component supports users in the triage, taking medical, radiological and resources aspects into account. A module allowing the resource management helps optimizing the strategy of application of the system.

A particular effort was made on the interoperability aspects of the software to easily exchange information with existing National Situation Awareness and Decision Support Systems. This interoperability is supposed to allow the integration of BOOSTER system in the existing crisis management plans and bring accurate information for Authorities of Civil Protection to take informed decisions.

Data transmission between the first level command and control nodes located near the incident site and the second level command and control centers located in a remote location are performed through large bandwidth means (high speed internet connections or satellite).

III. RED ZONE RESPONSE -INFORMING FIRST RESPONDERS

As the first phase of BOOSTER System deployment, a fast evaluation of the radiological situation is performed on the scene with newly developed and adapted existing measurement tools. The fusion of this information with the position of people in the scene allows an estimation of the dose received, may it be by irradiation or by contamination means.

A. A miniaturized gamma camera for hot spot location

As an example of the deployed components, the second generation gamma camera GAMPIX [START_REF] Gmar | GAMPIX: a new generation of gamma camera[END_REF], [START_REF] Carrel | GAMPIX: A new gamma imaging system for radiological safety and Homeland Security Purposes[END_REF], developed by CEA LIST and shown in Fig. 1, was adapted for use by first responders directly in the affected zone. Lightness and compactness of the system are therefore being emphasized along with the system temporal sensitivity.

The principal objective of this gamma camera is to locate the hot spots on the scene so that first responders could be aware of the dangerous areas and avoid them. This application requires, from the camera, to operate in a very large range of dose rates that could be encountered in emergency situations. A characterization of dose rate response of the gamma camera was carried out in a calibration laboratory at Karlsruhe Institute of Technology, Germany. Results on the 137 Cs source irradiator showed a good linearity in terms of dose rate assessment from 150 nGy/h to 23 Gy/h for point sources.

Specificities of the gamma camera have also been adapted to the type of radionuclides potentially present in a dirty bomb. Measurements with radiological sources of interest for security application were carried out at the Center for Energy Research of the Hungarian Academy of Sciences, Hungary. Results, some of them presented in Fig 2 , showed a good applicability of the gamma camera for a wide range of radioisotopes that could be encountered in the frame of a dirty bomb attack or a large scale radiological accident. It is important to note that the gamma camera was also found suitable with imaging extended sources, such as the Another application that is currently being studied is the detection of contamination spots on individuals to determine which part of the body might have been contaminated. This second field of use requires a good sensitivity to very low dose rates in addition to a good withdrawal of the ambient radiological noise by hardware and software methods. The sensitivity of the gamma camera was shown compatible with such a fast assessment. A 150 nGy/h source was imaged in less than 5 minutes.

Given presented results, the gamma camera was found suitable for both intended uses in the frame of BOOSTER system: first, the location of highly irradiating hot spots, to point out the hazardous areas to be avoided; secondly, the detection of heavy contamination spots on victims, as a prescreen before contamination assessment process.

B. Network of sensors for dose-rate mapping

In addition to this gamma camera, other sensors were assessed for being deployed over the affected area, allowing the evaluation of the situation. Those sensors, provided by AREVA CANBERRA, include compact dose rate meters equipped with a GPS module for mapping of radiation levels in the Exclusion area and portable HPGe detectors for identification of radionuclides faced [5]. Sensor platforms are adapted to each sensor to send the gathered information to a coordinated network based on the SIMACOP system developed by the Polytechnic University of Valencia, Spain. The core software processes the information from sensors and interpolates to get a picture of the radiological situation in the area of interest. Information from sensors, estimation from software and data from external sources are processed in a decision support system providing an evaluation of the exposure and contamination status of people, based on their position in the area.

IV. MITIGATION -INFORMING TRIAGE TEAM

A. A unique ID for individuals identification

Once getting out of the affected area, individuals are given a wristband with a unique barcode ID. This barcode follows the victim during the whole triage process. All analysis results or estimations from sensors are listed under this ID in a specific database that is reachable by triage staff and medical staff at any time for victims follow-up.

B. Tools for characterization of the external contamination

A complete set of tools was developed or adapted for the characterization of the external contamination. Decontamination tents are equipped with contamination monitoring portals, in order to determine the contamination status of each victim passing through. In case the contamination status turn to positive, the determination of the external contamination is supplemented with cutting-edge detection probes provided by AREVA CANBERRA which allow the detection and location of alpha, beta and gamma emitters. This information, processed by the central software of BOOSTER toolbox therefore allows determining the dose received by external contamination.

C. Tools for characterization of the internal contamination

New, lead and iron, low-background spectrometry systems, developed at the Center for Energy Research were made compatible with an in-field use. Those measurements were designed to be carried out on biological samples from victims (urine, saliva, nasal mucus…) to determine the activity of the incorporated gamma-emitting nuclides. Their design, based on several slices of less than twenty kilograms, allows them to be easily transportable in a van and deployed on the field for direct measurements. According to the first test results, the background radiation is decreased by about two to five magnitudes depending on the energy. Under emergency situation the iron chamber and lead container measurement systems should be used in such a location that the staff can operate the system under safe conditions. Based on these preliminary assessment and first measurements results, applying measurements of a few hundred seconds in such locations would lead to detect the presence of gamma radioactive material being under the exemption activity limit. For the most common isotopes, these values are activities from 1 kBq to 1 MBq and activity concentrations from 1 to 1.000 Bq/g, depending on the isotope of interest. By knowing the activity of radionuclides in the body and the approximate time of incorporation, the total dose received can be estimated. First results obtained with the developed low-background spectrometry system, applied to blood with incorporated isotopes, showed the possibility to detect, among others, down to 15 fg of 239 Pu, 5 ng of 238 U or 35 pg of 235 U.

In parallel, investigations on a portable Laser Induced Breakdown Spectroscopy (LIBS) lead to a detection limit of 242 ppm of 137 Cs in urine matrix. All together, these measurements allow the determination of the internal contamination of victims, at very low concentrations.

D. New biodosimeters for quick determination of received dose

Biodosimetric approaches are frequently focused on cytogenetics. The quantification of the dose from dicentric/ring chromosome scoring appears to be the most precise. Current optimization of such analysis involves the automation of various steps in the process. However, chromosome analyses are performed on the first mitosis post exposure and thus a significant limiting factor remains the culture step of two days that is impossible to reduce. New biodosimeters were developed in the frame of this project to accelerate the triage process.

Irradiation of an individual can be quickly determined by analysis of cellular processes that respond to ionizing radiation effects. Indeed, a major consequence of ionizing radiation is the generation of DNA double-strand breaks [START_REF] Rogakou | DNA Double-stranded Breaks Induce Histone H2AX Phosphorylation on Serine 139[END_REF]. These double-strand breaks induce a rapid and coordinated series of cellular responses, one of which is the phosphorylation of H2AX histone in large chromatin regions (up to 2 Mb) surrounding the break. These regions can be detected by immunofluorescence microscopy as subnuclear foci of phosphorylated H2AX (γH2AX) and the level of γH2AX foci is proportional to the number of nuclear double-strand breaks. Quantification of γH2AX thus allows the determination of the radiation dose received by the cells. However, this H2AX phosphorylation is time-dependent and also depends on individual radio-sensitivity. In order to achieve a new biodosimetric method, the property of the histone H2AX was used to develop a quick test to determine the level of irradiation of an individual. The test set up takes into account both the variability of the individual sensitivity and the delay for the observation for each individual. The aim is not only to estimate the dose, but also the risk, in order to define preventive actions, appropriate treatments, and subsequent storage place of the victims.

The Life Sciences Division of CEA tested the proteins extraction on pig hair and human blood which were irradiated at 0 Gy, 2 Gy, 4 Gy and 8 Gy. The phosphorylated histones H2AX (γH2AX) were quantified by labelling or by direct dosimetry of phosphorus with a LIBS system. It appeared that the γH2AX level was proportional to the irradiation doses as shown in Fig. 3.

E. Novel retrospective dosimetry on victim belongings

The main task of retrospective dosimetry is to assess or, if possible, to determine the absorbed dose posteriorly. Thermoluminescence (TL) and optically stimulated luminescence (OSL) are the most important analytical methods used in retrospective dosimetry. It is known, that natural materials such as sodium-chloride, dolomite or calcite, available in the environment, have luminescence properties and can be used for retrospective dosimetry [START_REF] Bailiff | The use of ceramics for retrospective dosimetry in the Chernobyl exclusion zone[END_REF], [START_REF] Bøtter-Jensen | Retrospective radiation dose reconstruction using optically stimulated luminescence on natural minerals[END_REF]. On the other hand the subservient luminescence properties of surfacemounted devices (SMD) have also been proved previously suitable for retrospective dosimetry purposes [START_REF] Inrig | Retrospective dosimetry using optically stimulated luminescence of electronic components[END_REF], [START_REF] Göksu | Telephone chip-cards as individual dosimeters[END_REF], [START_REF] Ekendahl | Retrospective dosimetry with alumina substrate from electronic components[END_REF]. Environmental materials were selected by the Center for Energy Research, for the investigations in BOOSTER project with respect to the characterization of their luminescence properties. Study focused on the complex characterization and application of SMD resistors for retrospective and accidental dosimetry due to their better properties for the purpose of the project. In addition to their very good properties, the main advantage of SMD resistors is their presence in every cell phone. This characteristic is convenient for having access to numerous retrospective dosimeters, since lots of people now have a cell phone.Consequently thermoluminescence properties and dose response of different types of SMD resistors were investigated. Several different types of resistors were investigated and measured. First results of dose response of different SMD resistors are presented in Fig. 4.

The following parameters of SMD resistors were investigated in the time: thermoluminescence glow curves, dose dependence, reproducibility, fading (stability) and energy dependence. All analysed parameters confirmed the suitability of the SMD devices for retrospective dosimetric purposes. Investigation of other properties (dose rate and temperature dependence, detection limits, etc.) is in progress. For laboratory measurements, most of the commercially available devices are suitable and capable to execute all the required steps. However, their physical parameters and design are not favourable for in-field applications. In the frame of the BOOSTER Project, a portable TL/OSL reader was especially designed by Hungarian partners for being used in-field.

F. Decision aiding component for triage

A software module, developed by the Karlsruhe Institute of Technology, is coupled to the command and control system to help triage teams in their mission and crisis manager to mitigate the situation as quickly as possible. This decision aiding component supports the user in categorising the priority of treatment of individual victims. Factors taken into account can be medical aspects (e.g. trauma patient against radiological exposure), radiological aspects (level of contamination) or resource aspects (number of victims and number of devices). This decision support system is based on the recommendations of the TMT handbook [12].

V. FURTHER FOLLOW-UP -INFORMING MEDICAL STAFF

A. Additional biodosimetric tools for refined dose

As an additional measure at medium-term post-exposure (hours to days), the scoring of centrosomes duplication will help to confirm quickly the victims triage. The levels of centrosome amplification occurring after irradiation were examined by the National University of Ireland, Galway as a biomarker for exposure [START_REF] Bourke | DNA damage induces Chk1-dependent centrosome amplification[END_REF]. New monoclonal antibodies were developed and characterized to quantitate centrosome number using microscopy and then these reagents applied for biodosimetry.

Irradiation caused a linear increase tendency in both the average number of centrosomes per cell from and in the population of cells with amplified centrosomes, confirming our finding that centrosome amplification is a dose-dependent response to DNA damage, as shown in Fig. 5 and Table II. Importantly, this analysis demonstrates that rapid, automated counting is feasible for centrosome quantification as a biomarker. It is also noteworthy that we can feasibly count large numbers of cells using microscopy, allowing improved measurement of centrosome numbers, which we expect to increase the sensitivity of this bioassay. Cells and centrosomes were imaged and quantified with the Operetta imaging system. The average number of centrosomes per cell (Fig. 5 A) and the percentage of cells with more than two centrosomes (Fig. 5 B) are shown. The increase in average number of centrosomes per cell and in the percentage of cells with amplified centrosomes correlated positively with the radiation dose (r 2 =0.996 and 0.999, respectively).

B. Granulometry for toxicity assessment

In addition to the radioactivity of the particles, it is essential to determine their granularity and their nature as, independently of radioactivity, toxicity will depend on the nature and the size of the particle. Small sizes are an aggravating factor, by allowing particles to reach more internal organs. The size of the particles is determined using Rayleigh inelastic diffraction or a simple diffraction, obtained by laser interference.

C. Laser Ablation Inductively Coupled Plasma Spectrometry for analyses of incorporated nuclides

Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is used for analysis of radionuclides incorporated in body fluids (saliva, blood and urine). Laboratory-based ICP-MS analyses are performed for more precise determination of the dose after the in-field triage. The analytical protocols which were developed by the Center for Energy Research for environmental and swipe samples were re-optimized for overcoming the potential matrix-related interferences during the analysis. Current methods allow the analysis of uranium and transuranium elements in body fluids; the estimated detection limits based on previous experiences on environmental and swipe samples are in the fg/g (µBq-mBq/g) range. The use of special sample introduction methods for direct sampling of solid materials by laser ablation allows extending the analytical capabilities of ICP-MS. Employing this method, the incorporation of elements (heavy metals, platinum group elements, uranium, etc.) can be investigated in the body. The analysis of a single hair with this approach allows the estimation of not only the time of incorporation, but also the change in the incorporated analyte during a given period.

VI. CONCLUSION AND FORESEEN IMPACT

The BOOSTER project focuses on developing new biodosimetric tools and integrating them with state-of-the-art measuring equipment and DSS software in a toolbox. The impact of a dirty bomb attack is well known: considerable disruptive effect, forcing public evacuations, creating economic chaos, and inflicting environmental and property damage with significant cleanup costs. The outputs of the BOOSTER project are expected to significantly improve the mitigation capability beyond the state of the art and propose new efficient tools for responders on field.

By performing a very fast triage, with newly developed biodosimetric tools and the complete set of radiological sensors, BOOSTER System allows to discriminate very quickly persons who need to be checked carefully from people who have no medical risks. It will also participate to ensuring that people do not overreact to the mere presence of radiation without full knowledge of the extent and type of contamination.

The new measurement tools also allow securing the first responder from the radiation exposure as these tools do not only check the affected people but also provide clear and comprehensive information on the existing contamination level which is beyond existing radiation equipment available so far.

The prototype of the BOOSTER System was successfully presented during a field trial in May 2013, on KFKI Campus, Budapest, validating the coupling of all devices and showing the applicability of such a system to intended crisis management. It is indeed important to note that even though it was developed for radiological attacks mitigation, the BOOSTER System is also applicable to the management of major accidents in nuclear facilities or transportation emergencies where individuals involved in the transportation and the nearby population might be exposed immediately.

  Manuscript received June 06, 2013. This work was supported in part by the European Commission under Grant SEC-GA-2009-242361. V. Schoepff, F. Carrel, M.Gmar, H. Lemaire, MF. Robbe are with the CEAvincent.schoepff@cea.fr). F. Carvajal and I. Perez Llopis are with the Universitat Politecnica de Valencia, Spain. D. C. Gaboriau and C. G. Morrison are with the National University of Ireland Galway (NUIG), Ireland. I. Almási, S. Szabo, A. Kelemen, A. Kovács, É. Kovács-Széles and D. Mesterházy are with the Centre for Energy Research, Hungarian Academy of Sciences (IKI), Hungary. K. Amgarou and N. Menaa is with AREVA CANBERRA, France. L. Morat, I. Testard, N. Ugolin and M. Viau are with CEA, DSV, F-92265 Fontenay-aux-Roses, France. F. Becker, W. Raskob and D. Trybushnyi are with the Karlsruhe Institute of Technology (KIT), Germany. A. Vincze is with the Hungarian Atomic Energy Authority (HAEA), Hungary.

  Fig 2 uranyl acetate.

Fig. 1 .

 1 Fig. 1. On the left: current prototype of the GAMPIX gamma camera. On the right: detection of a plutonium sample carried in a piece of luggage.

Fig. 2 .

 2 Fig. 2. On the top: uranyl acetate bottles. On the bottom left: plutonium source. On the bottom right: natural uranium bar. Detection duration and sources activities are voluntarily not stated.

Fig. 3 .

 3 Fig. 3. At the top: follow-up of H2AX phosphorylation in terms of intranuclear foci for irradiation of 0 Gy, 2 Gy and 4 Gy. At the bottom: scoring of phosphorylated H2AX after immunofluorescence microscopy analysis of human lymphocytes exposed to 1 Gy, 2 Gy and 4 Gy.

Fig. 4 .

 4 Fig. 4.On the left: picture of a SMD resistor. On the right: thermoluminescence glow curves of 100k type SMD resistor irradiated with different doses

Fig. 5 .

 5 Fig. 5. At the top: counting of amplified centrosomes 24 h after irradiations at 5 Gy and 10 Gy. At the bottom: quantification of centrosome amplification in DT40 cells after irradiation.

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ACKNOWLEDGMENT

The authors are very grateful to the team of experts and endusers that supported the project and gave their feedback on the system design, documents reviews and to participate to the final in-field exercise in Budapest. We would also like to thank Mrs. S. Pinto-Marques and Dr. M. Müller from Research Executive Agency for their support in the coordination and technical progress of this project.