N Decorbie

I Tijunelyte

S Gam-Derouich

J Solard

A Lamouri

P Decorse

N Felidj

C Gauchotte-Lindsay

E Rinnert

C Mangeney

N Lidgi-Guigui
email: nathalie.lidgi-guigui@univ-paris13.fr

Sensing polymer/paracetamol interaction with an independent component analysis-based SERS-MIP nanosensor

Keywords: Sensor, SERS, Molecularly Imprinted Polymers, Emerging pollutant, Paracetamol, Independent Component Analysis

Sensing polymer/paracetamol interaction with an independent component analysis-based SERS-MIP nanosensor

INTRODUCTION

Molecularly imprinted polymers (MIPs) are highly selective materials made of robust artificial polymer matrices which ensure long-term stability and can compete with biological receptors like aptamers or antibody [START_REF] Whitcombe | The rational development of molecularly imprinted polymer-based sensors for protein detection[END_REF][START_REF] Belbruno | Molecularly Imprinted Polymers[END_REF][START_REF] Ahmad | Watersoluble plasmonic nanosensors with synthetic receptors for label-free detection of folic acid[END_REF]. MIPs are obtained by the synthesis of crosslinked polymers in the presence of a template molecule which, after extraction, leaves complementary cavities in the film. Rebinding the template by the MIPs is highly specific and selective as the artificial receptors are shaped by the template [START_REF] Haupt | Molecularly Imprinted Polymers and Their Use in Biomimetic Sensors[END_REF]. The main figure of merit of MIPs compare to their biological counterparts include simple synthesis pathway and low cost, high stability to extreme chemical environment and remarkable reusability. They are especially appropriate for the detection of small molecular compounds; this is probably why their occurrence in environmental sciences is increasing (about 10% of the publications of MIPs for sensors according to web of science) [START_REF] Rico-Yuste | Molecularly imprinted polymerbased hybrid materials for the development of optical sensors[END_REF]. In the framework of sensing, MIPs are used mainly with electrochemical sensors but also with quartz microbalance [START_REF] Vasapollo | Molecularly Imprinted Polymers: Present and Future Prospective[END_REF]. While MIP-based sensors are a thriving research field, there are still only limited numbers of studies combining MIPs with plasmonic nanoparticles. In particular, their combination to lithographic nanoparticles has never been reported so far.

We propose here to graft MIPs onto gold nanostructures and to use this hybrid system as an original SERS (Surface-Enhanced Raman Spectroscopy)/MIPs hybrid sensor. SERS combines rich chemical fingerprint information based on Raman spectroscopy and high sensitivity due to the local field enhancement offered by optically resonant metal nanoparticles. This results in a strong amplification of the Raman signals and label-free identification of molecular species with high-sensitivity. Besides, the SERS analysis can be performed in aqueous medium under ambient conditions, which makes it well adapted for analytical chemistry. Considerable progress has been made in this field, from studies of model systems on roughened metallic surfaces, sensing and imaging applications, to single molecule detection [START_REF] Dick | Metal film over nanosphere (MFON) electrodes for surfaceenhanced Raman spectroscopy (SERS): Improvements in surface nanostructure stability and suppression of irreversible loss[END_REF][START_REF] Cottat | High Sensitivity, High Selectivity SERS Detection of MnSOD Using Optical Nanoantennas Functionalized with Aptamers[END_REF][START_REF] Tijunelyte | Diazonium salt-based surface-enhanced Raman spectroscopy nanosensor: Detection and quantitation of aromatic hydrocarbons in water[END_REF][START_REF] Schmid | Nanoscale Chemical Imaging Using Tip-Enhanced Raman Spectroscopy: A Critical Review[END_REF][START_REF] Qian | In vivo tumor targeting and spectroscopic detection with surfaceenhanced Raman nanoparticle tags[END_REF][START_REF] Zhang | Single Molecule Tip-Enhanced Raman Spectroscopy with Silver Tips[END_REF][START_REF] Nie | Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering[END_REF]. However, tremendous challenges remain in terms of reproducibility and sensitivity. To obtain highly sensitive SERS-active substrates, the most common strategies rely on disorganized nanostructures such as plasmonic colloidal films [START_REF] Delhaye | Tailoring surface-enhanced Raman scattering effect using microfluidics[END_REF], metallic nano-island layers on glass [START_REF] Járai-Szabó | Understanding selfassembled nanosphere patterns[END_REF][START_REF] Farcau | Mapping the SERS Efficiency and Hot-Spots Localization on Gold Film over Nanospheres Substrates[END_REF][START_REF] Cottat | Soft UV nanoimprint lithography-designed highly sensitive substrates for SERS detection[END_REF] or electrochemically roughened metallic surface [START_REF] Zhong-Qun Tian | Surface-Enhanced Raman Scattering: From Noble to Transition Metals and from Rough Surfaces to Ordered Nanostructures[END_REF]. While being simple to prepare and highly sensitive, these substrates offer poor reproducibility, originating from the complexity and heterogeneity of these irregular surfaces, which makes quantitative analysis a difficult task. Some strategies are, however, found to build organized nanostructured surfaces, for example by exploiting the organized array of nanorods left by a coffee stain [START_REF] Zhao | Plasmon-exciton coupling by hybrids between graphene and gold nanorods vertical array for sensor[END_REF] or using electroplating on top of a lithographied array [START_REF] Cetin | Fabrication of Sub-10-nm Plasmonic Gaps for Ultra-Sensitive Raman Spectroscopy[END_REF]. To overcome this issue and achieve more uniform SERS signals, plasmonic substrates with controlled nanostructuration were elaborated using Electron Beam Lithography (EBL). This approach provides a precise control over the geometry and separation of nanostructures, guaranteeing a fabrication reproducibility and precision down to the nanometer scale. The grafting of a probe molecule on a SERS substrate usually rely on thiol-based strategies. They exhibit excellent bonding affinity between sulfur and gold [START_REF] Cottat | High Sensitivity, High Selectivity SERS Detection of MnSOD Using Optical Nanoantennas Functionalized with Aptamers[END_REF][START_REF] Tripp | Novel nanostructures for SERS biosensing[END_REF][START_REF] Dribek | Organometallic nanoprobe to enhance optical response on the polycyclic aromatic hydrocarbon benzo[a]pyrene immunoassay using SERS technology[END_REF], however, unfavorable reaction conditions may deteriorate their recognition efficiency. Moreover, during the SERS measurement, the excitation of localized surface plasmon resonance may increase the surface temperature, leading to the desorption and photodecomposition of these chemoreceptor layers and of their thiol-metal links [START_REF] Schlenoff | Stability and Self-Exchange in Alkanethiol Monolayers[END_REF][START_REF] Jiang | Electrochemical Desorption of Self-Assembled Monolayers Noninvasively Releases Patterned Cells from Geometrical Confinements[END_REF]. This is why we have chosen here to use a more efficient strategy for the grafting of MIPs. Diazonium salt-based MIPs were synthesized to assure a good stability. We present an original hybrid MIP-SERS sensor easy to prepare, low-cost and reusable. Another of its advantage is that its all-solid form and the lack of treatment of the sample is suitable for in situ measurements. Paracetamol was chosen as the target molecule as it is a widely used drug with analgesic and antipyretic properties, considered nowadays as an emerging pharmaceutical contaminant [START_REF] Kasprzyk-Hordern | The occurrence of pharmaceuticals, personal care products, endocrine disruptors and illicit drugs in surface water in South Wales, UK[END_REF][START_REF] Kosma | Investigation of PPCPs in wastewater treatment plants in Greece: Occurrence, removal and environmental risk assessment[END_REF]. Owing to its intense use, high excretion rate, persistence and resistance to conventional wastewater treatments, it is found widespread in wastewaters, water effluents, and surface waters, in concentrations ranging from ng.l -1 to μg.l -1 . The sensitive detection of paracetamol is an important issue for several reasons. For instance, it can be found unmetabolized in urban wastewaters where the measurement of its concentration is fundamental for epidemiology analysis. A mapping of the concentration of paracetamol can be used to study the actual consumption of this drug by a population in a given area, just as it has already been done by the group J. B. Quintana on many molecules [START_REF] González-Mariño | Wastewater-Based Epidemiology To Monitor Synthetic Cathinones Use in Different European Countries[END_REF][START_REF] González-Mariño | Wastewater-Based Epidemiology as a New Tool for Estimating Population Exposure to Phthalate Plasticizers[END_REF]. Another point is that paracetamol, metabolized or not, can react with chemicals used in wastewater plants such as hypochlorite to form toxic compound [START_REF] Bedner | Transformation of acetaminophen by chlorination produces the toxicants 1,4-benzoquinone and Nacetyl-p-benzoquinone imine[END_REF]. Finally, paracetamol is part of a larger family of similar compound such as its metabolized compounds or even salicylic acid or acetyl salicylic acid. The process developed here stand as a proof of concept for all these molecules. To go one step further we have used numerical routine to discriminate between the three above molecules. Independent Components Analysis (ICA) was applied as a chemometric tool to analyze the paracetamol spectra. ICA is a statistical and computational technique for extracting source signals from their mixture. We show that it can be used to differentiate the spectra of the polymer with or without the drugs.

MATERIAL AND METHODS

Reagents

Paracetamol, methacrylic acid (MAA) and ethylene glycol dimethacrylate (EGDMA) were purchased from Sigma-Aldrich and used as received. 4-aminobenzophenone was obtained from Alfa Aesar. N,N-dimethyaniline, chloroform (CHCl3), methanol (MeOH), acetonitrile (ACN), dichloromethane (DCM) and acetone were all of analytical reagent grade from Acros Organics. Water was deionized using a Millipore purification system.

Synthesis of the initiator derived from diazonium salt C6H5-CO-C6H4-N2 + ,BF4 -

The initiator derived from diazonium salt was prepared by standard method of Pinson and co-workers[30].

Raman and SERS measurements

SERS and extinction measurements were recorded using Xplora ONE (Horiba scientific) spectrometer. For SERS, a 660 nm laser tuned at 3 µW was focused on the sample with x80 objective (N.A. 0.75). The used grating of 600 grooves.mm -1 ensured a spectral resolution of 3 cm -1 . Integration time for signal collection was set at 40s. Each spectrum was the average of two repetitive accumulations. The mean spectra presented here are the average of five spectra acquired on different location of the sample.

Chemometrics

As mentioned in the introduction, an ICA process was applied to the SERS spectra. This method decomposes a spectrum into linear combination of synthetic spectra called components and their coefficients are called scores. The estimated independent components (ICs) are often closely related to the spectral profiles of the individual chemical components in the initial mixture and thus chemically interpretable [START_REF] Hyvärinen | Independent component analysis: algorithms and applications[END_REF][START_REF] Rutledge | Independent Components Analysis with the JADE algorithm[END_REF]. Then a Partial least squares-discriminant analysis (PLS-DA) method was applied to analyze SERS spectra in the presence of different drugs. PLS-DA is a technique used for predictive and descriptive modelling as well as for discriminative variable selection. This method aims to maximize the covariance between SERS spectra and the corresponding classes (different drugs) by finding a linear subspace of the variables [START_REF] Brereton | Partial least squares discriminant analysis: taking the magic away[END_REF].

To facilitate the comparison among the spectra, the signal was normalized by dividing each spectrum by its Euclidean norm. All pretreatments and chemometric applications were performed using Matlab software R2017b (The MathWorks, Natick, USA).

MIP grafting on gold NC

Gold nano-cylinders arrays were elaborated on indium-tin-oxide (ITO) coated-glass substrates by electron-beam lithography (EBL). The functionalization of gold NCs arrays by MIPs was performed using a chemical strategy based on aryl diazonium salt initiators [START_REF] Nguyen | Engineering Thermoswitchable Lithographic Hybrid Gold Nanorods as Plasmonic Devices for Sensing and Active Plasmonics Applications[END_REF][START_REF] Nguyen | Looking for Synergies in Molecular Plasmonics through Hybrid Thermoresponsive Nanostructures[END_REF][START_REF] Ahmad | Tailoring the Surface Chemistry of Gold Nanorods through Au-C/Ag-C Covalent Bonds Using Aryl Diazonium Salts[END_REF]. These molecules were chosen as an alternative to alkanethiol self-assembled monolayers, in order to obtain stable and strongly attached layers at the surface of the plasmonic arrays.

The grafting process is summarized on Figure 1. A bifunctional initiator, 4-benzoylbenzenediazonium tetrafluoroborate, bearing (i) a diazonium end group for surface anchoring and (ii) a benzophenone function able to activate surface-initiated radical photopolymerization was synthesized according to ref [START_REF] Gam-Derouich | Electrografted Aryl Diazonium Initiators for Surface-Confined Photopolymerization: A New Approach to Designing Functional Polymer Coatings[END_REF]. The NCs were functionalized with the initiator-derived diazonium salts by simple incubation of the arrays in a water solution of the diazonium salt (3 mM) at room temperature for 2 h. After washing the plasmonic arrays with copious amount of water and ethanol, photopolymerization could proceed, incubating the NC arrays with a deoxygenated mixture of MAA (1 mmol) as the monomer, EGDMA (1.5 mmol) as the crosslinking agent, dimethylamine (DMA: 4wt.% relative to MMA and EGDMA)) as co-initiator and paracetamol (0.25 mmol) as the template molecule. Irradiation of the mixture under UV light for 4 h yielded the final products, consisting of NC arrays coated by molecularly imprinted polymers (Au@MIP). A reference non-imprinted polymer sample (Au@NIP) was prepared using the same procedure, but without the addition of the paracetamol template.

RESULTS AND DISCUSSION

Representative SEM images of the NCs are displayed in figure 2.

They exhibit a diameter D = 100 nm, a height H = 50 nm, and an interparticle distance Λ = 200 nm. The extinction spectrum of the NC array shows a main peak at 639 nm, corresponding to the localized surface plasmon resonance (LSPR). The plasmon band shifts to 682 nm after grafting of MIP and to 680 nm after washing the paracetamol from the MIP. This behavior is coherent with an increase of the local refractive index due to the grafting of the organic coating around the gold NCs and then a decrease when the paracetamol is removed. The shift of the LSPR with the paracetamol removal suggest that the layer is thin enough for the surface to be sensitive to chemical modifications. Electromagnetic simulations (can be found here: [START_REF] Colas | Comparison of adhesion layers of gold on silicate glasses for SERS detection[END_REF]) confirm that the volume in which the LSPR can detect any change is a few nanometer wide. The surface chemical composition of the NCs after functionalization was investigated by XPS. Only slight modifications of the spectra were observed after the reaction with the diazonium saltderived initiator, suggesting that the grafted initiator layer is very thin. No peak that could be attributed to nitrogen atoms from the diazonium group can be observed, evidencing the complete transformation of the diazonium cations. In contrast, the polymerization step induces strong variation of the XPS spectra, summarized in Table 1. The most striking observation is the progressive decrease of the gold signal and the relative increase of carbon and nitrogen due to the poly(MAA-co-EGDMA) coating. Table 1. Surface chemical composition (at. %) of initial Au NCs, Au@Benzophenone, Au@MIP and Au@NIP.

The samples were further characterized by SERS (see Figure 3). The Raman spectra were successively acquired on the bare NCs, the Au@MIP and the Au@NIP just after grafting and after washing the paracetamol away from the polymer. After washing, the Au@NIP spectra show variations reflecting modifications of the polymer matrix. For instance, the peaks at 1615 cm -1 and 1388 cm -1 are better defined once the Au@NIP has been rinsed. Similarly, the comparison between the Au@MIP spectra recorded before and after washing, evidences slight modifications. The peaks at 933 cm -1 , 1201 cm -1 , 1388 cm -1 and 1615 cm -1 cannot be seen when the paracetamol is in the matrix but appear clearly after washing. These peaks are comparable to those seen in the rinsed Au@NIP and can thus be assigned to the polymer matrix layer.

The spectrum of the paracetamol does not emerge. This can be explained by the fact that the vibrational modes of the drug are strongly affected by the polymer matrix. Also, the paracetamol concentration is very likely too low compared to the polymer one.

The decrease or disappearance of some of the MIP's peaks (1201 cm -1 peak for instance) reveals the growing interaction between the polymer and paracetamol, and thus the presence of the contaminant.

Figure 3. SERS spectra of the bare cylinders (black); Au@MIP before (red) and after (light red) washing away the paracetamol; Au@NIP before (dark green) and after (light green) washing the polymer. The brown curve is the Raman spectra (without NC) of the paracetamol powder.

Increasing concentrations solutions of paracetamol were added to the Au@MIP and Au@NIP samples in order to analyze the evolution of the SERS peaks intensity (Figure 4). As observed above, the paracetamol molecular fingerprint cannot be seen in the Au@MIP spectra. However, some of the peaks of the Au@MIP decrease in intensity as the concentration in paracetamol increases, especially the 2 prominent peaks at 1073 cm -1 and 1579 cm -1 . The spectra of the Au@NIP show minor variation as the polymer is exposed to increasing concentrations of paracetamol. From this observation, a calibration curve has been derived and is shown on figure 5. The intensity of the peak at 1073 cm -1 (I1073) was plotted for each concentration. As expected, I1073 was observed to decrease for Au@MIP with increasing concentration of paracetamol. The experimental points were then fitted to measure the sensitivity of the sensor. The absence of a plateau shows that even at 1 mM, the SERS-MIP sensor is not saturated. The limit of detection (LoD) (i.e. the smallest reasonable value detected) is given by the intersection of the calibration curve and the curve of equation <xbl> + k , where <xbl> is the mean intensity of the blank sample,  is its standard deviation and k is a parameter reflecting the confidence level of the detection. The LoD of the SERS-MIP sensors is given by the intersection between the green and red curve and it is less than 300 nM (45 g.l -1 ). As paracetamol is an emerging contaminant that has been detected in different environmental water at concentrations ranging from 50 to 80 g.L -1 , [START_REF] Żur | Organic micropollutants paracetamol and ibuprofen-toxicity, biodegradation, and genetic background of their utilization by bacteria[END_REF] the characteristics of the Au@MIP sensor described here, appear well adapted for detection of paracetamol in real sample concentration ranges.

The SERS analysis described above was further confirmed by means of multivariate analysis. An Independent Component Analysis (ICA) was used on the rough paracetamol spectra. On the top part of Figure 6, each point represents the score of the first component of each spectrum. The Au@MIP (point 1 to 30) have clearly a lower score than the Au@NIP (point 31 to 60). The difference between the two kinds of samples is thus confirmed by the ICA analysis. In order to test the selectivity and the reusability of the sensor, a series of consecutive measurements was made on the same Au@MIP sample. After a first SERS measurement, the sample was washed and measured again. The procedure was repeated with an exposition to salicylic acid and acetylsalicylic acid (two molecules with similar structures to paracetamol). The result of the PLS-DA analysis is shown on the bottom part of figure 6. The score obtained for the rinsed sample is clearly higher than those obtained in presence of a drug. The red dot line of figure 6 is the calculated discrimination of the empty MIP. The score obtained for the salicylic and acetylsalicylic acids are of the same order while the score of paracetamol is lower, confirming the selectivity of the nanosensor towards paracetamol.

CONCLUSION

In conclusion, we have designed and produced a hybrid sensor made from the grafting of a MIP on gold nanocylinders. This sensor was able to detect paracetamol in water with a limit of detection of 300 nM. It also presents a good sensitivity and selectivity. The sensor has demonstrated a good reproducibility and specificity. The use of multivariate analysis confirmed the results obtained by classical analysis. The results presented here are a proof of concept that the detection of emerging pollutants such as paracetamol is possible in a routine way. Other results have demonstrated that it was possible to take advantage of the polarization of the light to perform a regio-selective functionalization of the nanocylinders [START_REF] Tijunelyte | Multifunctionalization of lithographically designed gold nanodisks by plasmon-mediated reduction of aryl diazonium salts[END_REF]. These complementary results pave the way to ultra-sensitive multiplexed SERS sensors.

Figure 1 .

 1 Figure 1. Schematic illustration of the surface modification strategy for coating the gold NC arrays with a MIPs layer, by combining diazonium salt chemistry and photopolymerization.

Figure 2 .

 2 Figure 2. Characterization of the gold NC array. (a) SEM image of the nanoarray with cylinders of 100 nm in diameter and a pitch of 200 nm. (b) Normalized extinction spectra showing the LSP of the NCs: (black) before MIPs functionalization, (red) after functionalization and (blue) after washing the paracetamol.

Figure 4 .

 4 Figure 4. SERS spectra of the MIP (thick lines) and NIP (thin lines) exposed to increasing concentration of paracetamol

Figure 5 .

 5 Figure 5. Calibration curve from the intensity of peak 1073 cm -1 . Black dot: experimental point and green line fit. Red line: LoD

Figure 6 .

 6 Figure 6. ICA analysis of the SERS spectra. Each point represents a spectrum. Up: the first component of the analysis makes a good separation between the MIP (point 1 to 30) and the NIP (point 31 to 60). Bottom: the analysis of the same samples in the presence of different drugs using PLS-DA. The red dot line shows the discrimination between the paracetamol and the other state of the polymer

Figure 1 .

 1 Figure 1. Schematic illustration of the surface modification strategy for coating the gold NC arrays with a MIP layer, by combining diazonium salt chemistry and photopolymerization.

Figure 2 .

 2 Figure 2. Characterization of the gold NCs array. (a) SEM image of the nanoarray with disks of 100 nm in diameter and a pitch of 200 nm. (b) Extinction spectra showing the LSP of the NCs: (black) before MIP functionalization, (red) after functionalization and (blue) after washing the paracetamol.

Figure 3 .

 3 Figure 3. SERS spectra of the bare cylinders (black); Au@MIP before (red) and after (light red) washing away the paracetamol; Au@NIP before (dark green) and after (light green) washing the polymer. The brown curve is the Raman spectra (without NC) of the paracetamol powder.

Figure 4 .Figure 5 .Figure 6 .

 456 Figure 4. SERS spectra of the MIP (thick lines) and NIP (thin lines) exposed to increasing concentration of paracetamol

ACKNOWLEDGEMENTS

The authors would like to thanks the C'Nano for its financial support in the framework of the SYRENSS project.


Issam Kherbouche

D Macrae

Théo Geronimi

F Lagugné-Labarthet

Azedine Lamouri

Alexandre Chevillot Biraud

Claire Mangeney
email: claire.mangeney@parisdescartes.fr

Nordin Félidj
email: nordin.felidj@univ-paris-diderot.fr

Extending nanoscale patterning with multipolar surface plasmon resonances †

Plasmonic excitation of metallic nanoparticles can trigger chemical reactions at the nanoscale. Such optical effects can also be employed to selectively and locally graft photopolymer layers at the nanostructure surface, and, when combined with a surface functionalization agent, new pathways can be explored to modify the surface of a plasmonic nanoparticle. Among these approaches, diazonium salt chemistry is seen as an attractive strategy due to the high photoinduced reactivity of these salts. In this work, we demonstrate that it is possible to trigger the site-selective grafting of aryl films derived from diazonium salts on distinct nano-localized area of single gold nanotriangles, by taking advantage of their multipolar localized surface plasmon modes. It is shown the aryl film will preferentially graft in areas where the electric field enhancement is maximum, independently of the considered excited surface plasmon mode. These experimental findings are in very good qualitative agreement with the calculations of the local electric field, using the finite-difference time-domain (FDTD) method. We believe that this plasmonic-based approach will not only pave a new way for the spatially controlled surface functionalization of plasmonic nanoparticles, but also provide a general strategy to attach distinct molecules to hot spot regions on a single nanoparticle, opening promising prospects in sensing and multiplexing, and optically nano-scale patterning of functional groups.

Introduction

Plasmonic nanoparticles with subwavelength size exhibit remarkable optical properties since they can support localized surface plasmon (LSP) resonances. 1 These LSP resonances are related to a collective oscillation of conduction electrons at the particle surface, the frequency of which depends on the size, the shape and the chemical composition of the particles, the inter-particle distance, and the refractive index of the surrounding medium. [2][3][4] LSP excitation results in an enhanced extinction in the far-field, mostly located in the visible and near-infrared spectral range for metals such as gold and silver, and a huge enhancement of the local electromagnetic field at the particle surface. [START_REF] Maier | Plasmonics: fundamentals and applications[END_REF][START_REF] Hao | [END_REF][7] Therefore, plasmonic nanoparticles are particularly important for their high potential in non linear optics, 8 chemo-sensors 9,10 or substrates for surface-enhanced spectroscopies, such as surface-enhanced Raman scattering (SERS). 11,12 In the last decade, it has been shown that metal nanoparticles can also act as reactive elements due to their ability to generate hot carriers, through plasmon decay. 13,14 When nanoparticles are in the vicinity of molecular compounds, hot electrons are extracted from the metal into available molecular orbitals of the adsorbates (i.e., injection of electrons into an anti-bonding state), causing, for instance, the dissociation of a bond in the molecular adsorbate. [15][16][17] It has been shown that the transient ions can then react on the metal surface. 18 Through such plasmon-induced processes, many reactions have been controlled at the particle scale, such as the dissociation of H 2 or other photocatalytic reactions some of them being of societal importance such as water splitting and carbon dioxide reduction. 19,20 . Interestingly, the mapping of the particle reactivity at the nanometer scale can be achieved, following precisely the spatial distribution of the electric field enhancement, under LSP excitation. 18,21 In the context of plasmon-induced chemical surface functionalization, a major challenge exists in the ability to trigger the site-selective grafting of functional layers on distinct nanoscale areas around plasmonic nanostructures. Recently, we developed an original route, based on aryl diazonium salt chemistry, to spatially control the surface functionalization of gold nanostructures under LSP excitation 22 . These molecules present the advantage to create strong covalent Au-C bonds with the surface, promoting stable interfacial links between the nanoparticles (NPs) and the organic functional layers. 23 In addition, they offer a wide range of terminal functional groups for future post-functionalization reactions [START_REF] Luo | Chemical Communications[END_REF][START_REF] Therien | [END_REF] . The grafting mechanism relies on a two-step process involving (i) the reduction of diazonium salts yielding reactive aryl radicals able to attach covalently to the surface and (ii) the reaction of the aryl radicals on the already grafted layers. 23 The thickness of the polyaryl layers can thus be monitored from monolayers to multilayers up to a few tens of nanometers. 26 Using plasmon-driven reduction of aryl diazonium salts, it is possible to pattern the surface chemical properties of lithographic gold nanodisks with different types of functional polyaryl layers, through a change of the incident light polarization (from 0 to 90°). 26 Interestingly, the grafting takes place specifically in the regions where the local field is enhanced. 27,28 Compared to isotropic shapes, triangular and pyramidal nanoparticles display even more intense electromagnetic fields due to their sharp corners. [29][30][31] In addition, their tunable optical properties as a function of the edge length can lead to various multipolar LSP modes at distinct wavelengths, with a high concentration of electromagnetic field at different locations of the triangular nanoparticles as well as the arrays composed of adjacent nanotriangles. 31,32 Therefore, such triangular-shaped particles could provide various spatially resolved grafting areas, via the excitation of distinct multipolar LSP modes. 33 To our knowledge, the excitation of diverse multipolar LSP modes in order to give access to a wide range of reactive nanoscale areas for the versatile site-selective surface functionalization of gold NPs has never been reported so far. In this work, we address this issue by considering regular arrays of gold triangular nanostructures with spectrally resolved in-plane multipolar LSP modes. Such structures, designed through electron beam lithography (EBL), display distinct intense bands in the visible and near infrared spectral range, depending on their lateral size. 33 Herein, we focused our attention on the first three lowest energy multipolar LSP modes, corresponding to the dipolar, quadrupolar and sextupolar LSP modes. These three modes are particularly interesting since they are characterized in the nearfield by strong field enhancements located at distinct places, depending on the excited multipolar LSP mode. Plasmon-induced grafting of diazonium salts has proved to be very efficient in the case of salts bearing various functionalities such as carboxylic acid or hydroxyethyl, which can be easily post-functionalized, as already demonstrated in previous papers. 26 The diazonium salt which was used to induce the plasmon-mediated grafting of aryl films is 4-nitrobenzenediazonium tetrafluoroborate (4NBDT). 34 By taking advantage of the multipolar localized surface plasmon modes, we explored the possibility to expand the limits of surface chemistry by confining the grafted aryl layers in a wide variety of nanoscale regions of maximum electric field enhancement around the gold nanostructures, as schematized in the figure 1 (and displayed later in Figs. 3, 4,and6). The experimental findings were compared to the calculations of the local electric field performed using the finite-difference time-domain (FDTD) method, 35 in order to reveal the close relationship between near-field enhancement and surface grafting. We believe that this plasmonic-based approach will not only pave an easy way for the functionalization of plasmonic nanoparticles, but also provide a general strategy to attach molecules to hot spot regions and further improve their SERS detection and analysis for (bio)sensing applications. 36 Fig. 1 Scheme of the strategy of multipolar plasmon-mediated surface functionalization. The location of the aryl film grafting will depend on the excited multipolar plasmon mode. It is expected that the grafting will take place specifically on the area sustaining the maximum intensity of local electric field. It will be mainly located at the tips of the triangle for a dipolar excitation (left), on the lateral sides for a quadrupolar excitation (center), and slightly on the edges and the apexes at the basis of the triangle for the sextupolar excitation (right).

Results and discussion

Gold nanotriangle arrays were fabricated by electron beam lithography in order to control precisely their LSP wavelengths. 37,38 An SEM image of such structures, referred as array A, is shown in the Fig. 2a. It corresponds to a square array of gold equilateral triangles, with edge lengths of 170 nm, a thickness of 50 nm, and an interparticle distance of 340 nm (grating constant, center to center). The experimental optical response of the array revealed two distinctive LSP bands (Fig. 2b,spectrum A): a dominant one at 822 nm, identified as the dipolar LSP mode (1 st order), and a less intense one at 614 nm, attributed to the quadrupolar LSP mode (2 nd order). The lower intensity observed in the case of the quadrupolar mode, compared to the dipolar one, is due to the specific distribution of oscillating charges limiting a re-radiation in the far-field optical response. The experimental extinction spectrum was compared to the calculated extinction cross section using the FDTD method (Fig. 2b, spectrum B), highlighting a good qualitative agreement between the experimental and theoretical far-field optical response.

The plasmon-induced grafting of aryl layers derived from diazonium salts was performed by simply immersing the nanostructures in a diazonium salt aqueous solution (3 mM), and irradiating the sample during 10 seconds using the appropriate laser. The 785 nm laser line was used to excite the dipolar mode, with maximum extinction resonance located at 822 nm, while the quadrupolar mode at 614 nm was excited using the 633 nm laser line. The optical exposure was performed in normal incidence, and the laser was focused on the array through an immersion microscope objective (100× numerical aperture N.A. 1), resulting in a circular laser spot of 3 µm diameter at the surface. Light polarization was tailored in order to be along the base of the triangles (X-polarization), or perpendicular (Y-polarization). After the optical exposure, the grafting was characterized using scanning electron microscopy (SEM), and surface-enhanced Raman spectroscopy (SERS). 39 Fig. 3(a) and Fig. 3(c) display the SEM images of the particles after an exposure of 10 sec. with the 785 nm laser and a polarization along the X-axis and Y-axis, respectively. The laser power was set at 0.13 mW/µm 2 , corresponding to an energy dose, defined as the product of the laser power by the optical exposure time, of 1.3 mJ/µm 2 . Remarkably, independently of the incident polarization, the SEM images clearly show a nano-localized grafting of organic layers. For the X-polarization, the organic spots are observed mainly at the two apexes of the base of the triangles with identical spatial extent on both sides. In contrast, for the Y-polarization, the grafted layers appear mostly at the top of the triangles. It is noteworthy that the grafting is uniform over the whole sample, as shown in the Fig. SI 1 displaying a SEM image containing more triangles, than the ones showed in Fig. 3(c). Although the mechanism of grafting is still in debate in the literature, it has been shown theoretically that the probability of chemical reactions (such as bonding, or chemical reduction,. . . ) is maximum precisely where there is a maximum of electric field enhancement. 18 A thermal process alone is excluded since it would have had the effect of depositing the organic layer over the entire surface of the particle. The chemical nature of these organic patches has been characterized by SERS, confirming the presence of polyaryl layers derived from nitrobenzene diazonium salts, as shown in the SI (Fig. SI 2a). In addition, after each aryl film grafting, we systematically observed a red-shift of the LSP band, attributed to the increase of the refractive index of the surrounding medium due to the presence of the aryl film (see Fig. SI 2b).

In order to interpret these results, the local electric field distribution was calculated by the FDTD method, with an incident illumination at 803 nm corresponding to the maximum of the dipolar LSP band). The mapping of the local electric field intensity displays maxima situated mainly at the apexes of the structures, with a location depending on the incident polarization. For a polarization parallel to the base of the triangle (X-polarization), the two maxima are located at the apexes located on the base (Fig. 3b). For a perpendicular polarization (Y-polarization), one maximum appears on the apex on the top of the triangle and two other enhanced electric field regions are observed on the base apexes (Fig. 3d). The comparison between the spatial extent of the grafted layers observed by SEM ( Fig. 3a and Fig. 3c) and the mapping of the calculated electric field intensity around the triangles (Fig. 3b and Fig. 3d) emphasizes that the distribution of the polyaryl patches is a clear replica of the dipolar near-field intensity.

The quadrupolar mode could also be used to trigger the nanolocalized grafting of polyaryl layers derived from diazonium salts, in different nanoscale area. Fig. 4a displays the SEM image of the triangles after an exposure time of 120 sec. with the 633 nm laser line, an incident Y-polarization, and a laser power of 0.06 mW/µm 2 . It shows that the aryl patch distribution is radically different from that observed under excitation of the dipolar mode at 785 nm (Fig. 3), with organic layers mainly observed at the three edges of the triangles. As evidenced in Fig. 4b, the spatial extent of the grafting nicely follows the mapping of the electric field intensity using the FDTD method, for an incident illumination at 600 nm, related to the excitation of the quadrupolar LSP mode. It is worth noting that both X-and Y-polarization of incident light result here in comparable patterns (Fig. 4a and Fig. 4c) due to the similarity of the electric field intensity distribution of these configurations (Fig. 4b and Fig. 4d). In contrast to the dipolar mode, the choice of the incident polarization (X-or Y-polarization) for the quadrupolar mode does not change significantly the location of the aryl film.

In order to confirm that the location of the plasmon-induced grafting is not governed by the excitation line, but rather by the excitation of the distinct LSP modes, two new arrays of triangles were designed. The first one (array B) with a dipolar LSP mode matching the laser line at 633 nm, is made of equilateral triangles, with edge lengths of 90 nm, a thickness of 50 nm, and a Fig. 4 (a) SEM image of the triangles (array A), after immersion in an aqueous solution of diazonium salt (3 mM) and optical exposure at 633 nm for a polarization along the Y axis; (b) mapping of the intensity of the incident electric field at 600 nm, calculated by the FDTD method for a triangular target with 170 nm side length, height 50 nm, and a grating constant of 340 nm (polarization along the Y axis); (c) SEM image of the triangles, after optical exposure at 633 nm for a polarization along the X axis; (d) FDTD mapping of the intensity of the incident electric field at 600 nm, for the same triangular target (polarization along the X axis) -bar scale: 50 nm.

grating constant of 400 nm (see inset of Fig. 5a). As a result, a single LSP band is emerging at 668 nm in the extinction spectrum recorded in water, associated with the dipolar mode (see Fig. 5a, spectrum B). This is in accordance with the FDTD calculations, showing a single extinction cross section band at 690 nm, and a mapping of the electric field associated with a dipolar character (Fig. 5b, and Fig. SI3). It is thus expected that, for a laser line at 633 nm, the excitation of the dipolar mode induces an aryl film grafting at the apexes of the triangles, and not at the edges. The experimental results obtained after an exposure time of 60 sec. at 633 nm, with an energy dose of 0.5 mJ/µm 2 and a polarization along the Y-axis, confirm this hypothesis, with polyaryl patches appearing exclusively at the apexes of the triangles (Fig. 5c). For the array A (spectrum A), an excitation at 633 nm, upon excitation of its quadrupolar mode, leads to a site-selective organic coating located at the edges, and not at the apexes (Fig. 4a and Fig. 4b).

We considered an additional array (array C), displaying a quadrupolar mode close to 785 nm. This array C was made of equilateral triangles, with edge lengths of 250 nm, a thickness of 40 nm, and a grating constant of 400 nm (Fig. 6a,inset). As a result, a LSP band is emerging (in water) at 740 nm in the extinction spectrum, attributed to the quadrupolar mode (Fig. 6a). For a density of energy of 9 mJ/µm 2 at 785 nm, and a polarization along the Y-axis, the aryl film grafting is located at the edges of the triangles, in agreement with the excitation of the quadrupolar mode (Fig. 6b). The location of the grafting nicely follows the mapping of the electric field intensity associated to the quadrupolar mode calculated at 725 nm (Fig. 6c), and can be compared to the mapping of array A, for which the grafting, at 785 nm, fol- lows the mapping of the dipolar mode (Fig. 3). It can thus be concluded that the location of the aryl film grafting is only governed by the order of the chosen multipolar mode, and not by the laser line.

Interestingly, the array C displays an additional LSP band located at 590 nm, attributed to the 3 rd order of the LSP mode, the so-called sextupolar mode. The mapping of the associated calculated electric field reveals a slightly distinct electric field distribution (calculated at 610 nm), compared to the dipolar and quadrupolar ones, with an assymetric distribution of the electric field along the lateral edges, and two maxima at the apexes of the triangle base (Fig. 6e). To investigate the possibility to induce site-selective surface grafting of aryl layers under excitation of this sextupolar mode, array C was immersed in the diazonium salt solution and exposed to irradiation at 633 nm, close to the wavelength of the sextupolar mode. With a density of energy of 7 mJ/µm 2 and a polarization along the Y-axis, yielded a distinctive pattern with a maximum grafting at the apexes of the triangles, and an asymmetric distribution along the edges (Fig. 6d and6e). The location of the grafting thus qualitatively follows the mapping of the electric field associated to the sextupolar mode.

Some differences have to be pointed out between the location of the aryl film grafting and the electric field mapping for the sextupolar mode (Fig. 6d and Fig. 6e). The grafting along the edges is not as obvious, in comparison with the mapping of the electric field, althougth it slightly appears at the top of the two edges. However, the two other grafting locations are clearly observed at the two apexes of the base of the triangles, in good qualitative agreement from the FDTD calculations. Despite the poor matching conditions of the excitations wavelengths, these preresonances conditions appear effective to photoinduce the onset of spatially controlled polymerization that mimic the FDTD mapping calculated at the resonances. This is clearly shown in 6b and Fig. 6c for the quadrupolar mode and for 6d and Fig. 6e for the sextupolar mode. Better matching conditions would presumably yield more effective spatial localization with shorter irradiation or lower irrandiance. It can be also noted that the grafting, through the sextupolar mode, leads to a pattern similar to the one obtained for a dipolar mode. This similarity precisely points out the versatility of our strategy: being able to position the organic layer at any locations, regardless the size of the triangles.

One critical aspect deals with the control of the thickness of the aryl film layer, which is of main importance in the context of biosensing applications or surface-enhanced spectroscopy analyses to adjust the distance between the particle surface and the probed analytes. 40 In order to tune the organic patch thickness and spatial extent, the energy dose was varied, from 0.69 to 8.3 mJ/µm 2 . The SEM images of array A, after quadrupolar modeinduced grafting (at 633 nm) and a dipolar mode illumination (at 785 nm) respectively, are displayed in Fig. 7a at increasing energy doses. An average thickness was measured as the distance between the particle surface and the upper end of the thicker area of the aryl film layers versus the energy dose for both modes. From these SEM images, it appears that the thickness of the grafted organic layers increases progressively with the energy dose, inducing a higher degree of polymerization, while remaining strongly confined in intense electric field area. As reported in Fig. 7b, the average film thickness reaches ca. 50-60 nm at high energy dose (8.3 mJ/µm 2 ) with a tendency to plateau, at least for the dipolar mode. This plateau is attributed to the fact that the near-field enhancement is maximum at the particle surface, but decreases rapidly exponentially with the distance from the surface. 26 Interestingly, it can be noted that for a given incident energy dose, the layer thickness is systematically higher for a dipolar excitation, compared to the quadrupolar one. This trend can be attributed to the fact that the intensity of the dipolar electric field is higher than the intensity of the quadrupolar one, for a given intensity of the incident electric field. [START_REF] Hao | [END_REF] The thickness of the polyaryl film can thus be controlled by both the energy dose and the selected multipolar LSP mode.

Conclusion

In summary, we have shown that the excitation of multipolar LSP modes expands the possibilities of plasmon-driven surface functionalization with aryl diazonium salts and opening new routes of surface chemistry and patterning. By playing on various multipolar modes, different chemical patterns could be accessible, from the nano-localized grafting of organic patches exclusively on the apexes of triangular NPs upon dipolar mode excitation, to the confinement of organic layers specifically on the edges by illuminating the quadrupolar mode. Interestingly, the sextupolar mode could also be used to trigger a different chemical pattern. By systematically comparing the experimental results with the mapping of the electric field intensity calculated by the FDTD method, it appears that the multipolar plasmon-induced grafting occurs specifically in the regions of maximum field enhancement, leaving the other areas of the nanostructure's surface chemically passive. In addition, the thickness of the organic patches can be tailored by changing the energy dose and the selected LSP mode, from few nm up to 50-60 nm, opening promising prospects for nanosensing.

Experimental section

Gold nanotriangle arrays were fabricated by electron beam lithography in order to achieve a precise monitoring of their LSP wavelengths, and to ensure that the LSP modes match well with the available excitation lines at 633 nm (He-Ne laser), and 785 nm (Laser diode). For their fabrication, a 190 nm thick layer of poly(methyl methacrylate) (PMMA) electron resist was spin coated on glass substrates covered with a 80 nm layer of transparent conducting indium tin oxide (ITO). In the exposure step, the substrates were exposed to an electron beam which was scanned over the sample. Chemical development, thermal vacuum coating with gold and a lift-off procedure followed, which led to regular arrays of gold triangular structures exhibiting 35-50 nm height and variable edge lengths and inter-particle distances (grating constant, center to center).

The diazonium salt which was used to induce the plasmonmediated grafting of aryl films is 4-nitrobenzenediazonium tetrafluoroborate (4NBDT). The plasmon-induced grafting of aryl layers derived from diazonium salts was performed by simply immersing the nanostructures in a diazonium salt aqueous solution (3 mM), and irradiating the sample at various times using the a 633 or 785 nm laser lines. The optical exposure was performed in normal incidence, and the laser was focused on the array through an immersion microscope objective (100× numerical aperture N.A. 1), resulting in a circular laser spot of 3 µm diameter at the surface.

The LSP resonance of the samples was probed by farfield visible-NIR extinction micro-spectroscopy in the range of 500-900 nm, with irradiation by using a halogen lamp at normal incidence from the glass side. The spectrometer was coupled to an optical microscope equipped with a 50× objective (numerical aperture N.A.0.75). The investigated area was a circle of approximately 80 µm diameter, which was covering the entire array (100×100 µm 2 ). The spectra were recorded in water, since the LSP-induced grafting of the diazonium salt takes place in this medium.

The Raman and SERS experiments were carried out by using a Jobin-Yvon LABRAM HR 800 Raman micro spectrometer. The incident sources are a He-Ne laser (632.8 nm), and a laser diode (785 nm) focused on the sample, through a microscope equipped with a 100× objective (Olympus, NA 0.8).

Finite-difference time-domain (FDTD) modelling (Lumerical FDTD Solutions) was used to simulate the extinction cross-section and the electric field at the surface of the triangular nanoparticles. The gold triangular nanoparticles were of size and thickness as indicated. The structures were modelled on a substrate of refractive index 1.78 to match the ITO substrate, 41 and the surrounding medium of refractive index of 1.33 to match the surrounding water. 42 The gold was modelled using the dielectric functions of gold from Palik. [START_REF] Palik | Handbook of optical constants of solids[END_REF] To match the array fabricated experimentally, periodic boundary conditions were set along the X-and Y-boundaries, set to match the grating constant of the array. In order to truncate the simulation area, perfectly matched layers were applied along the Z-boundary. The structures were meshed by a grid of 2 nm in the X-, Y-and Z-directions. A maximum of 10 mesh cells per wavelength was applied outside of the triangular nanoparticles.

Fig. 2

 2 Fig. 2 (a) SEM image of lithographic gold equilateral nanotriangles (170 nm side length, height 50 nm, grating contant 340 nm) over a glass substrate covered by a thin layer of ITO; (b) Far-field optical response (in water): experimental extinction spectrum (spectrum A); extinction cross section calculated by the FDTD method (spectrum B). The laser lines are indicated as vertical dashed lines (at 633 and 785 nm).

Fig. 3

 3 Fig. 3 (a) SEM image of the triangles (array A: 170 nm side length, height 50 nm, and grating constant 340 nm), after immersion in an aqueous solution of diazonium salt (3 mM) and optical exposure at 785 nm for a polarization along the X axis; (b) mapping of the intensity of the electric field at 803 nm for a polarization along the X axis, calculated by the FDTD method for a triangular target with 170 nm side length, height 50 nm (bar scale: 50nm); (c) SEM image of the same array of triangles after an optical exposure at 785 nm for a polarization along the Y axis; (d) FDTD mapping of the intensity of the electric field at 803 nm or a polarization along the Y axis (bar scale: 50nm).

Fig. 5

 5 Fig. 5 (a) Experimental extinction spectrum of equilateral triangles: spectrum A, with edge lengths of 170 nm, a thickness of 50 nm, and grating constant of of 340 nm (in inset, SEM image of a single triangle), and spectrum B, with edge lengths of 90 nm, a thickness of 50 nm, and grating constant of of 400 nm (in inset, SEM image of a single triangle). The spectrum is recorded in water; (b) FDTD mapping of the intensity of the electric field at 690 nm, and a polarization along the Y axis for the triangle of 90 nm edge; (c) SEM image of a 90 nm edge triangle, after immersion in an aqueous solution of diazonium salt (3 mM) and optical exposure at 633 nm for a polarization along the Y axis.

Fig. 6

 6 Fig. 6 (a) Experimental extinction spectrum in water of gold nanotriangles (array C): lateral size: 250 nm, height: 40 nm, grating constant: 400 nm. Inset: SEM image of the corresponding triangles; (b) SEM image of a triangle, after immersion in an aqueous solution of diazonium salt (3 mM) and optical exposure at 785 nm for a polarization along the Y axis (exposure conditions: 9 mJ/µm 2 ). (c): mapping of electric field intensity at 725 nm irradiation, calculated by the FDTD method. Scale bar: 50 nm; (d): SEM image of a triangle, after optical exposure at 633 nm for a polarization along the Y axis (exposure conditions: 7 mJ/µm 2 ); (e): mapping of electric field intensity at 610 nm irradiation, calculated by the FDTD method. Scale bar: 50 nm.

Fig. 7

 7 Fig. 7 (a) SEM images of the array A after immersion in aqueous diazonioum salt solution (3 mM), and optical exposure at various energy doses ranging from (1) 0.69 mJ/µm 2 , (2) 2.08 mJ/µm 2 (3) 4.1 mJ/µm 2 to (4) 8.3 mJ/µm 2 Exposures were made at 633 nm, corresponding to the excitation of the quadrupolar mode (top frame surrounded in red), and at 785 nm, corresponding to the excitation of the dipolar mode (bottom frame surrounded in blue); (b) Average thickness, measured as the distance between the particle surface and the upper end of the thicker area of the aryl film layers versus the energy dose for the dipolar (blue squares) and quadrupolar (red squares) LSP modes.

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J o u r n a l N a me , [ y e a r ] , [ v o l . ] ,

Acknowledgements

The author thanks ANR-IDEX "double culture" (Université de Paris) for its financial support.

Notes and references

Conflicts of interest

There are no conflicts to declare.


Da Li

Théo Geronimi

Nordin Felidj

Philippe Nizard

Delphine Onidas

Yun Luo

Claire Mangeney

Surface modification of plasmonic nanomaterials with aryl diazonium salts

This chapter provides an overview of the various surface modification strategies based on the use of aryl diazonium salts for the functionalization of plasmonic nanoparticles. Aryl diazonium salts appear as a valuable alternative to commonly used thiol selfassembled monolayers due to several advantages, including the formation of robust interfacial metal-surface bonds, simple synthesis and surface grafting protocols, a large range of functional groups and the possibility to form multilayers around the nanoparticle surface. Owing to these outstanding features, the combination of aryl diazonium salts and plasmonic nanoparticles is receiving increasing attention for potential applications in various fields such as nanosensors, bioimaging, environment, forensic science and antimicrobial materials. This chapter describes the chemical processes involved in the surface modification of plasmonic nanoparticles by diazonium salts, the resulting nanohybrids and the targeted applications.

Properties of plasmonic nanoparticles

Plasmonic nanoparticles (NPs) with unique chemical, physical and optical properties have attracted a lot of attention over the past decades. These NPs sustain a collective coherent oscillation of the free electrons at their surface upon electromagnetic excitation, called localized surface plasmon (LSP), as illustrated in Fig. 1. [1] When the frequency of the incident radiation matches the frequency of LSP modes, this results in LSP resonance (LSPR), which can be observed as intense extinction bands in the visible and nearinfrared spectral range. The plasmon band wavelength depends on the NPs' composition, size, shape and surrounding dielectric environment. Therefore, the presence of analytes in the vicinity of plasmonic NPs can be detected by spectral shifts of the LSP wavelength, induced by changes of the local medium refractive index, allowing real-time chemical and biological sensing. The excitation of LSP is also accompanied by the generation of highly enhanced electromagnetic fields, which have been widely used in surfaceenhanced spectroscopies, such as Surface-Enhanced Raman spectroscopy (SERS) to detect trace amount of analytes, down to the single-molecule level. [2] Besides, nonradiative relaxation of LSP results in heat energy release and production of hot carriers. The plasmonic NPs can thus act as nanosources of heat for photothermal applications or charge carriers (hot electrons and holes) to induce chemical reactions locally. [3] Among the different types of plasmonic NPs, gold and silver NPs (Au NPs and Ag NPs) have been the most widely studied. While the size and shape of the NPs are crucial parameters to optimize their optical properties, the control of their surface chemistry is also a critical issue as it determines the interaction between the NPs and their environment. Therefore, considerable efforts have been made in order to develop appropriate surface modification strategies, with the aim to strengthen colloidal stability and to add new functionalities, thereby broadening the application scope of plasmonic NPs.

Common strategies for surface functionalization of plasmonic NPs

The main strategies for surface functionalization of plasmonic NPs are based on either physical adsorption or covalent bonding. Compared to physical adsorption, covalent bonding offers more robust interactions between the NPs and the immobilized molecules, thus improving interfacial stability. The most common strategy consists in using thiol ligands resulting in the formation of thiolate self-assembled monolayers (SAMs), anchored on the NPs surface via gold-sulfur bonds. [4] Thanks to the rich chemistry of gold-sulfur bonds, a wide variety of functional groups have been attached around the plasmonic cores leading to innovative hybrid nanomaterials. [5] However, issues regarding the chemical stability of SAMs have been identified. For example, they have been shown to desorb spontaneously from surfaces after few days in aqueous solutions. [5,6] Moreover, they degrade upon laser excitation [7] and can be exchanged in biological media, jeopardising their efficacy and reliability. Although significant improvements have been obtained via the use of longer-chain thiols, multidentate sulfur-based adsorbates, silica coatings or small amounts of amphiphilic surfactants, the development of alternative approaches to obtain strong covalent anchoring of surface functions onto the plasmonic cores still remains an important challenge.

Surface functionalization of plasmonic NPs with aryl diazonium salts

A recent approach has emerged for the functionalization of plasmonic nanomaterials based on the use of aryl diazonium salts. General formula of aryl diazonium salts can be expressed as A -, + N2-C6H4-R, where A -is the counter-ion and R stands for the chemical function on the aromatic ring. Since Jean Pinson and coworker's pioneering work in 1992 described the mechanism of electrochemical reduction of aryl diazonium salts to modify glassy carbon electrodes, [8] these compounds have attracted widespread attention in the field of surface chemistry. Indeed, the cleavage of N2 from diazonium salts occurs at low potential and on reducing surfaces opening promising prospects as surface functionalization agents in various applications including nanomedicine, forensic science, sensors, catalysis and energy. These compounds offer several advantages over other common strategies, such as their ease of synthesis, their fast grafting on surfaces, the large choice of available functional groups and the formation of robust covalent interfacial bonds with the surface. Generally, their grafting on NPs involves a two-stage mechanism. Firstly, the decomposition of the diazonium salts generates an aryl radical able to bind on the surface. In a second step, the formed highly reactive radicals react with the already grafted groups resulting in nanometer to micrometer thick polyaryl layers. To obtain functionalized NPs, diazonium salts can be used either as isolated compounds or can be generated in situ. The use of isolated salts allows to control the initial concentration of the reactants. However, the long-term stability of the diazonium salt solids and solutions is a major concern. The use of in situ generated aryldiazonium salts from the corresponding aromatic amines can overcome this issue and achieve high grafting densities on the NPs while preserving colloidal stability. Several approaches have been proposed to modify plasmonic nanomaterials by diazonium salts. [9] The main ones based on electrochemistry, addition of reducing agents, spontaneous reaction or plasmonmediated chemistry, are described below (Fig. 2).

Electrografting

As aryl radicals can be generated by electrochemical route, electrografting of aryl diazonium salts has become a powerful method to functionalize supported plasmonic NPs. For example, arenediazonium tosylates could be deposited on plasmon-active Au and Ag thin films via either spontaneous reactions or electrochemistry (see Fig. 3). [10] Interestingly, the spontaneous grafting results in thin monomolecular layers with potential interest for Surface Plasmon Resonance (SPR) technique or SERS sensors while electrochemical activation generates thick polyaryl layers opening promising prospects for applications in tunable plasmonic devices and plasmon-based lasers. In another example, aryl diazonium salts were electrografted via pulse deposition and used as antifouling coatings onto Au-based SPR sensors. [11] The antifouling performances of the coatings obtained from diazonium salts bearing different functional groups or alkanethiolate self-assembled monolayers were compared, revealing an enhanced antifouling behavior in the case of 4-phenylalanine diazonium chloride with a significant 1 For the sake of clarity, the scheme (b) depicts only the case of the simultaneous reduction of auric and isolated diazonium salts. The mechanisms based on the phase transfer of diazonium cations to [AuCl4] -phase or on the reduction of benzenediazoniumtetrachloroaurate(III) complexes by sodium borohydride are not represented.

Addition of reducing agent

reduction of the nonspecific adsorption surface coverage of crude bovine serum proteins (down to 42 ± 4 ng/cm 2 ). Proteins, modified with an aryl-diazonium adduct, could be also electroadressed onto gold SPR imaging (SPRi) chips for the detection of antibodies and ovalbumin. [12] This approach yielded multiple spots of robustly anchored proteins distributed over the SPRi biochip for the detection of antigen/antibody interactions.

Spontaneous grafting or chemical reduction

Contrary to macroscopic surfaces for which electrografting of diazonium salts is mostly used, it becomes tricky in the case of colloidal NPs. Therefore, for colloidal NPs, spontaneous reactions or addition of a reducing agent in the medium appears as simple alternatives to generate radicals from aryldiazonium salts. The spontaneous grafting is a straightforward approach consisting to mix the NPs with the diazonium salts and to let it react at room temperature, in the presence of air, as described by McDermott et al. [13]. This simple process was proved to preserve the colloidal stability of the nanohybrids and to yield strongly anchored organic shells with the desired functionality on the NP surface.

The possibility to obtain, with this approach, multifunctional layers with various chemical groups around the plasmonic cores, as illustrated in Fig. 4, was assessed using SERS, with the presence of different tags around the NPs. [14] Au NPs functionalized by aryl groups were also obtained via the phase transfer of diazonium cations to [AuCl4] - phase, [15] the simultaneous reduction of auric and isolated diazonium salts [16] or through the reduction of the benzenediazoniumtetrachloroaurate(III) complexes by sodium borohydride (NaBH4). [17] The resulting NPs were shown to withstand long periods of sonication in various solvents and remained perfectly stable in the solid state, in ambient conditions. (magenta), Au@NO2 (blue), Au@CN@NO2 (green) and Au@NO2@CN (red), and their corresponding Raman spectra. Adapted from ref. [14]. Copyright, 2020, Royal Society of Chemistry.

Plasmon-mediated grafting

Another approach to reduce aryldiazonium salts at the surface of plasmonic NPs consists of taking advantage of the "hot electrons" produced by the excitation of surface plasmons. Hot electrons are generated in nano-localized area around the NP surface, sustaining a strong near field enhancement, resulting in site-selective hot-electron mediated reduction of diazonium salts. This plasmon-mediated surface functionalization approach thus offers an unprecedented "bottom-up" approach to place molecules or nanomaterials into reactive area or high electromagnetic field regions (hot spots). It was used for the regioselective functionalization of plasmonic nanostructures, including nanotriangles, [18] nanorods, [19,20] nanodisks [21]. For example, aryl layers derived from bisthienylbenzene diazonium salts were regioselectively grafted on gold nanotriangles under visible-light illumination (Fig. 5a), in the absence of reducing agent. [18] Spatially selective grafting could be also achieved on colloidal gold nanorods using both iodonium and diazonium salts. Plasmon-assisted homolysis of the C-I bond of iodonium salts under excitation of longitudinal plasmon resonances resulted in the regioselective grafting of organic layers on the tips of the rods while their sides were functionalized using spontaneous diazonium salt grafting. [20] In another example, lithographic gold nanorods were irradiated by a polarized laser beam along their long axis, resulting in the preferential grafting of polyaryl layers on the nanorod tips (Fig. 5b,c). [19] Interestingly, a multi-step process using successive illuminations under perpendicular polarization directions, in the presence of two different diazonium salts, enabled the formation of multifunctional patterned layers grafted at the surface of gold nanodisks. [21]

Surface immobilization of small molecules, polymer films and biomolecules at the surface of plasmonic nanoparticles

Triggering the surface immobilization of functional species on plasmonic NPs is of great importance for advanced applications in (bio-)sensing, environment, catalysis and optics. The different strategies based on diazonium salts to attach small molecules, polymer coatings and biomolecules on the surface of metallic NPs are summarized below (Fig. 6).

Surface immobilization of small molecules

Small molecules (with weight less than 900 daltons) based on aryl diazonium salts, [START_REF] Macielag | Antibiotic Discovery and Development[END_REF] can be grafted onto metallic NPs via reduction of diazonium salts. The resulting organic mono-or multi-layers attached on the NPs have been considered as promising interfaces for various applications, such as chemical and bio-sensors or bioimaging tags. For example, macrocyclic compounds, such as calix [4]arenes bearing diazonium groups on their large rim were grafted on Au NPs. [START_REF] Troian-Gautier | [END_REF] Remarkably, these molecules can be decorated with various functions, such as oligo(ethylene glycol) chains providing an excellent colloidal stability to the nanohybrids. Mixtures of calixarenes could be also grafted on the NPs, with a fine control over the final surface chemical composition, opening the way to the design of Au NPs coated with well-defined functional and postfunctionalizable groups. [24] Thanks to this strategy, carboxyl groups from PEG layers were conjugated to amine-containing biomolecules such as peptide aptamers, for the detection of cancer biomarkers. [25] Regarding bioimaging applications, the grafting of Raman reporter groups at the surface of plasmonic NPs is an effective means to design SERS tags. Among the possible Raman reporter groups, nitro-and cyanophenyl moieties are particularly attractive as they display intense and characteristic Raman peaks, which can be easily detected by Raman imaging. These Raman labels were spontaneously grafted onto Au NPs for Raman imaging of tumor cells after NPs internalization. [14]

Surface immobilization of polymers

The development of strategies to obtain plasmonic NPs with surface-grafted polymers has stimulated a wide interest for potential application in (bio-)sensors [26], drug release [27] or environmental remediation. They can be classified into two main categories: (i) polymers can be "grafted to" the metallic NPs by adding a diazonium end group on the backbone of pre-formed polymer chains. For example, calix [4]arene-tetra-diazonium bearing four oligo(ethylene glycol) chains on their small rim [24] were used to functionalize Au NPs and provide them water solubility, steric stabilization and increased circulation lifetime in the blood; (ii) the polymerization can be driven "from" the surface where active species initiate the covalent polymerization. "Grafting from" polymerization techniques offer an accurate control of polymer coating thickness, homogeneity, and grafting density. Different polymerization mechanisms have been initiated from the surface of plasmonic NPs, including atom transfer radical polymerization (ATRP) [28], reversible addition/fragmentation chain transfer polymerization (RAFT) and polymerization controlled by an iniferter. Interestingly, the surface-initiated growing of stimuli-responsive (thermo-or pH-sensitive) polymer brushes, such as poly(Nisopropylacrilamide) (PNIPAM) by ATRP on plasmonic nanostructures was shown to yield active optical hybrid devices, which properties can be modified via an external stimulus. [29] Another strategy was based on plasmon-activated "from surface" RAFT controlled radical polymerization. [30] The RAFT agent was grafted on gold gratings via aryl diazonium salts and plasmon excitation was used to trigger surface-assisted growth of PNIPAM. The process appeared to be self-limited offering new prospects for the formation of ultrathin smart polymer coatings on gold nanostructures. The iniferter method from surface-grafted diazonium salts-derived initiators was exploited to immobilize molecularly imprinted polymers on gold nanorods to yield water-soluble plasmonic nanosensors [31] or on nanostructured copper surfaces to obtain anti-icing coatings. [32]

Surface immobilization of biomolecules

The flexible chemistry provided by the substituent on the aryl group offers many opportunities to immobilize on plasmonic NPs a wide range of biomolecules modified by diazonium end groups or aryl layers for post-functionalization. For example, Au NPs bioconjugates with insulin [START_REF] Albab | Colloids and Surfaces A: Physicochemical and Engineering Aspects[END_REF] or bovine serum albumin (BSA) [START_REF] Hameed | [END_REF] were elaborated using aryl diazonium salt chemistry, demonstrating a high hemocompatibility with human red blood cells. Au NPs modified by aryl diazonium salts bearing carboxylic acid groups were coated with polydiallyldimethylammonium chloride, [35] a biodegradable cationic polyelectrolyte to form conjugates with deoxyribonucleic acid (DNA) and obtain nonviral gene systems. Capture antibodies, such as anti-B-type natriuretic peptide (anti-BNP), could also be immobilized on Au NPs grafted through aryl diazonium salt chemistry to obtain sandwich type immunoassays on screen printed carbon electrodes for the accurate detection of clinically relevant levels of BNP in human serum samples. [36] In another strategy, the amino groups of Au NPs modified by 4-aminothiophenol were transformed to diazonium salts in order to bind a tumor marker, the human p53 protein.

The nanoconjugates were then captured on a sensor surface modified by another protein, azurin, which enables the formation of stable complexes with p53 protein. The transduction process was based on the SERS activity of Au NPs, resulting in intense SERS signals in the presence of p53 proteins in human serum, even at low concentrations (down to 500 fM). [37]

Targeted applications

Thanks to the outstanding properties of plasmonic NPs and their easy surface functionalization by aryl diazonium salts, a considerable scope for application research is open up, as illustrated in Fig. 7. It will take advantage of the enhanced colloidal stability of plasmonic NPs provided by functionalization with diazonium salts, robust interfacial links between the inorganic cores and the organic coatings and the wide variety of available functional groups. The targeted applications cover a broad range from i) biomedical and anti-bacterial science, ii) environmental protection, iii) forensic science and iv) catalysis.

Biomedical science

The use of plasmonic NPs in biomedical science has been the subject of intense research. In this field, the NPs must be modified with a variety of functional (bio-)molecules, such as drugs, targeting groups, chemical tags or antibacterial agents. For example, Au NPs were modified via aryl diazonium salts by BSA conjugates for drug delivery applications [START_REF] Hameed | [END_REF]. A remarquable hemocompatibility was observed after surface modification with BSA offering promising opportunities for intravenous drug delivery, as illustrated in Fig. 7a. Nanosensors based on molecularly imprinted polymers (MIPs) could be elaborated by combining aryl diazonium salts, iniferter polymerization and gold nanorods for the detection of analytes by SERS. The gold nanorods coated by MIPs layers enabled the SERS label-free detection of folic acid down to 0.1 M concentration. [41] Optical transduction based on SERS was also reported for the detection of p53 proteins in human serum at low concentrations [37]. In another approach, Au NPs were trapped within nanocomposite films of MIPs deposited on gold electrodes. [42] For this, immobilized Au NPs were functionalized with benzophenone, via aryl diazonium salts, to initiate the photopolymerization of methacrylic acid and ethylene glycol dimethacrylate, in the presence of dopamine, used as the template molecule. The resulting nanocomposite layers were used for the electrochemical label-free detection of dopamine, down to 0.35 nM concentration. Regarding antimicrobial applications, silver NPs were mainly used for their capacity to release Ag + ions, demonstrating strong antibacterial properties. Nevertheless, Ag NPs suffer from colloidal instability and tend to aggregate in aqueous solutions, thereby affecting their antibacterial effect. Aryl diazonium salt chemistry was exploited to improve their colloidal stability by introducing surface-grafted carboxylic groups acting as water-soluble stabilizers, resulting in efficient antibacterial nanohybrids towards S. Aureus. [43]

Environmental protection

Nanocomposites were elaborated from the electrochemical reduction of [HOOC-4-C6H4N≡N]AuCl4 salts in the presence of polyaniline (PANI) emeraldine salt for the removal of synthetic dyes in textile wastewater treatment. The composites were shown to exhibit a high removal rate of methylene blue dye onto nanosand paving the way towards environmental remediation applications. [44] Diazonium salts and Au NPs were also used as building blocks for the preparation of hybrid thin films on indium tin oxide (ITO) electrodes to obtain heavy metal sensors. Square wave voltammetry (SWV) measurements showed a high sensitivity for the electrochemical detection of Cu 2+ ions, with a linear behavior between mM to pM concentrations and a detection limit of about 1 pM. [45] In another example, diazonium salt-decorated Au NPs were used to detect aniline, an important raw material in many industries, which presence in waters raises pollution concerns. The presence of aniline resulted in a fast and visible red-to-blue color change of the Au NPs solutions (Fig. 7c) allowing naked-eyes detection of arylaminepolluted water and quantitative measurements by absorbance changes. [39]

Forensic science

Diazonium salts combined with metallic NPs were evaluated as innovative developers for latent fingerprints on crime tools. [46][40] This strategy, based on the in situ reduction of aryldiazonium [4-NO2-C6H4N2] + AuCl4 by the components of human sweat, allowed the formation of gold deposits on eccrine secretions present at the surface of copper, lead, nickel and aluminum surfaces. The gold deposits appeared efficient to visualize the primary, secondary, and tertiary details of the developed fingerprints. This approach was combined to the XPS technique, [47] a surface-sensitive technique for chemical analysis. The formation of robust gold-aryl films from simultaneous reduction of diazonium moieties and gold(III) in [AuCl4] -resulted in an increased gold(0) quantity on the surface of copper coins and model flat sheets. Diazonium gold(III) salts thus provide a promising alternative to common methods for latent fingerprint detection and development.

Catalysis

Water-soluble polyaryl-coated Au NPs were used as catalysts for the reduction of an environmental pollutant, 4-nitrophenol (Fig. 7b). It is noteworthy that nitrophenols are toxic organic pollutants widely spread in industrial and agricultural waste waters. Nanohybrids elaborated from the mild reduction of [HOOC-4-C6H4NN]AuCl4 displayed high catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol by sodium borohydride. [38] A series of para-substituted phenyl groups (-CH3, -F, -Cl, -OCF3, and -CF3) derived from diazonium salts were successfully grafted on the surface of Pt NPs, by co-reduction of diazonium salts and H2PtCl4. [48] -CF3 functionalized Pt NPs were shown to display the highest catalytic activity for oxygen reduction reactions (ORR), with  3 times higher ORR specific activity than the value found for commercial Pt/C catalysts. In another example, palladium NP-cored G1 dendrimers obtained from the reduction of Pd II in the presence of diazodendrons were used for the chemoselective room temperature hydrogenation catalysis of carbon-carbon multiple bonds, with good recyclability and short reaction times. [49] The Pd-G1 nanohybrids appeared to be also efficient catalysts for Suzuki, Stille and Hiyama coupling reactions. [50] Table 1 summarizes the main characteristics of the various plasmonic nanomaterials functionalized by aryl diazonium salts described in this review, and their targeted applications.

Conclusion

This book chapter provides a general overview of the various surface modification strategies based on aryl diazonium salts for the functionalization of plasmonic nanomaterials. Aryl diazonium salts appear as a valuable alternative to commonly used thiol self-assembled monolayers for the modification of plasmonic nanoparticles. Indeed, they offer added-value characteristics such as (i) friendly synthesis and surface grafting protocols, (ii) broad choice of available terminal groups, (iii) robust covalent surface-C interfacial bonds and (iv) formation of multifunctional layers. Owing to these outstanding properties, surface agents based on aryl diazonium salts were applied to the functionalization of plasmonic NPs for a wide range of applications, including nanosensors, nano-optics, bioimaging, latent fingerprints, environment and antimicrobial materials.

However, this research area is still on an early stage with respect to real-life applications and several challenges need to be addressed to promote industrial development. From a methodological point of view, the mechanisms of covalent binding on the plasmonic NP surface and formation of polyaryl multilayers should be fully elucidated to achieve wellcontrolled nano-hybrids. In practical terms, the long-term stability of the modified nanomaterials in various environments and upon external stimulations, such as light, temperature or pH should be examined in depth. From a toxicological standpoint, the biosafety of plasmonic NPs functionalized by diazonium salts should be assessed. To tackle these issues and boost the technology transfer of diazonium salt chemistry for plasmonic NPs functionalization, it will be crucial to establish coordinated and collaborative investigations covering chemical, toxicological and industrial aspects.

Figure 1 .

 1 Figure 1. (a) Representative scheme of the collective oscillation of free electrons at the surface of plasmonic NPs. The excitation of LSP results in a cascade of effects, including: (b) a strong electromagnetic (EM) field enhancement which can be exploited for SERS, (c) the generation of hot carriers and (d) heat dissipation leading to local heating.

Figure 2 .

 2 Figure 2. Representative scheme of the main approaches for the surface modification of plasmonic NPs by aryldiazonium salts, via (a) electrografting; (b) addition of a reducing agent 1 ; (c) spontaneous reaction between the diazonium salt and the NPs and (d) plasmon-mediated reduction of diazonium salts.

Figure 3 .

 3 Figure 3. Representative scheme of the functionalization of plasmon-active Au and Ag thin films by arenediazonium tosylates: spontaneous reactions or electrochemistry. Reproduced with permission from ref.[10]. Copyright, 2017, Elsevier.

Figure 4 .

 4 Figure 4. (a) Representative scheme of the spontaneous grafting of aryl diazonium salts on Au NPs and formation of multifunctional layers; (b) Raman images of dried drops of Au@CN(magenta), Au@NO2 (blue), Au@CN@NO2 (green) and Au@NO2@CN (red), and their corresponding Raman spectra. Adapted from ref.[14]. Copyright, 2020, Royal Society of Chemistry.

Figure 5 .

 5 Figure 5. (a) SEM image of Au triangles after plasmon-induced grafting during 4 min under visible light irradiation, in the presence of bisthienylbenzene diazonium salt aqueous solution; (b) Result of the difference between AFM images of Au nanorod recorded before and after plasmonmediated grafting under illumination polarized along the long axis; (c) Representative picture of gold nanorod modified via regioselective surface functionalization under plasmon excitation with polarized light. (a) Reproduced and readapted with permission from ref. [18] Copyright, 2017, American Chemical Society. (b) and (c) Reproduced and readapted with permission from ref. [19]. Copyright, 2017, Royal Society of Chemistry.

Figure 6 .

 6 Figure 6. Representative scheme of the surface immobilization of small molecules, polymer layers and biomolecules at the surface of plasmonic NPs, via diazonium salt chemistry.

Figure 7 .

 7 Figure 7. (a) Representative scheme of the effect of Au-BSA bioconjugates upon hemocompatibility and comparison with uncoated Au-COOH NPs. (b) UV-vis spectra recorded during the reduction of 4-NPh with NaBH4 in the presence of AuNPs-COOH. The inset shows photos of the solution during the reduction reaction from 0 to 5 min. (c) UV-vis spectra (left) and photos (center) of Au NPs functionalized by diazonium salts, in the absence (top) and presence (bottom) of aniline (100 μM). TEM images of the corresponding Au NPs solutions in the absence (up) and the presence (down) of aniline (100 μM). Scale bars: 50 nm. (d) Latent fingerprint image obtained using diazonium solution on a nickel coin. (a) Reproduced and readapted with permission from ref. [34]. Copyright, 2018, Elsevier. (b) Reproduced and readapted with permission from ref. [38]. Copyright, 2019, Royal Society of Chemistry. (c) Reproduced and readapted with permission from ref. [39]. Copyright, 2017, Elsevier. (d) Reproduced and readapted with permission from ref.[40]. Copyright, 2021, Wiley.

Table 1 .

 1 Summary of plasmonic NPs functionalized by aryl diazonium salts and their targeted applications.

	Type of NPs	Shape of NPs	Functional groups on aryl rings	Immobilized objects	Targeted application	Ref.
	Au	sphere	-NO 2 , -CN, -COOH	Aryl mono-or multi-layers	SERS tags for cell labeling	[14]
	Au	sphere	-C 8 F 17	Polyaryls	-	[17]
	Au	sphere	-Br	PNIPAM brushes	optical sensors	[28]
	Au	nanorod	-O-CH 2 -CH 2 -DEDTC	Molecularly imprinted polymers	Capture and by SERS detection of FA	[31]
	Au	nanorod	-C 2 H 4 OH	Polyaryls	Regioselective	[19]
	Au	nanodisk	-COOH, -C 2 H 4 OH	Polyaryls	grafting for nano-optics	[21]
	Au	nanotriangle	-BTB	Polyaryls	Plasmonic electrochemistry	[18]
	Ag	spheroid	-COOH	Polyaryls	Antimicrobial applications	[43]
			-CH 3 , -F, -		Electrocatalysis
	Pt	sphere	Cl, -OCF 3 , -	Polyaryls	for oxygen reduction	[48]
			CF 3 ,		reactions
					Chemoselective
	Pd	sphere	dendron	G 1 -dendrimer	hydrogenation	[49]
					catalyst