Haneul Yoo

Junghyun Shin

Jieun Sim

Hyunmin Cho

Seunghun Hong
email: seunghun@snu.ac.kr

Reusable surface plasmon resonance biosensor chip for the detection of H1N1 influenza virus

Keywords:

We developed a reusable magnetic surface plasmon resonance (SPR) sensor chip for detecting various target molecules repeatedly in a conventional SPR system. Here, ferromagnetic patterns on a SPR sensor chip were utilized to trap a layer of magnetic particles, and they were utilized as a solid substrate for SPR sensing in a conventional SPR system. After a sensing experiment, the used magnetic particles were removed by external magnetic fields, and a new layer of magnetic particles was immobilized to the SPR sensor chip for additional sensing measurements. Since magnetic particles were trapped on the ferromagnetic patterns, we could use our reusable SPR chip for SPR sensing measurements in a traditional SPR system without any applied magnetic fields. Significantly, ferromagnetic patterns on the sensor chip surface deflected the strong external fields, so that the large aggregation of magnetic particles on the sensor surface was reduced. We demonstrated using a single reusable SPR sensor chip to measure the nucleoprotein (NP) of H1N1 influenza virus solution ranging repeatedly for more than 7 times without significant signal degradation. Also, different target molecules could be repeatedly measured in a single SPR chip. Since our reusable SPR sensor chip can be repeatedly used in a conventional SPR system without any chemical processes for refreshment, the cost for SPR sensing should be significantly reduced. In this case, our reusable SPR sensor chip can be a major breakthrough and can be used for versatile practical applications of SPR sensors.

Introduction

A surface plasmon resonance (SPR) based sensing method is a powerful tool for the real-time and label-free monitoring of biomolecular interactions [START_REF] Krishnamoorthy | Surface plasmon resonance imaging based multiplex biosensor: integration of biomolecular screening, detection and kinetics estimation[END_REF][START_REF] Mandenius | Monitoring of influenza virus hemagglutinin in process samples using weak affinity ligands and surface plasmon resonance[END_REF][START_REF] Wang | Review: advances and applications of surface plasmon resonance biosensing instrumentation[END_REF][START_REF] Wong | Surface plasmon resonance imaging sensors: a review[END_REF][START_REF] Zeng | Nanomaterials enhanced surface plasmon resonance for biological and chemical sensing applications[END_REF]. In SPR-based biosensors, the selective bindings of target molecules to receptors on the SPR sensor surface induced its refractive index changes, which can be measured for the label-free detection of versatile target analytes in real time [START_REF] Wang | Review: advances and applications of surface plasmon resonance biosensing instrumentation[END_REF][START_REF] Wong | Surface plasmon resonance imaging sensors: a review[END_REF]. Also, various SPR-based measurement systems have been used extensively for the analyses of interaction kinetics between different biomolecules such as protein-ligand, protein-protein, or DNA-protein [START_REF] Jecklin | Label-free determination of protein-ligand binding constants using mass spectrometry and validation using surface plasmon resonance and isothermal titration calorimetry[END_REF][START_REF] Karlsson | Experimental design for kinetic analysis of protein-protein interactions with surface plasmon resonance biosensors[END_REF][START_REF] Patching | Surface plasmon resonance spectroscopy for characterisation of membrane protein-ligand interactions and its potential for drug discovery[END_REF][START_REF] Pollet | Fiber optic SPR biosensing of DNA hybridization and DNA-protein interactions[END_REF][START_REF] Wegner | Fabrication of histidine-tagged fusion protein arrays for surface plasmon resonance imaging studies of protein-protein and protein-DNA interactions[END_REF]. However, most of SPR biosensor chips are rather expensive compared with common commercial biosen sors such as strip-type sensors. Also, it is usually very difficult to reuse a used SPR sensor chip. Such a limitation has been one of the major stumbling blocks holding back its diverse commercial applications.

Recently, several groups have developed methods for reusable SPR sensors [START_REF] Choi | Reusable biosensors via in situ electrochemical surface regeneration in microfluidic applications[END_REF][START_REF] Hsu | In situ regeneration of Si-based ARROW-B surface plasmon resonance biosensors[END_REF][START_REF] Lee | Surface plasmon resonance biosensing based on target-responsive mobility switch of magnetic nanoparticles under magnetic fields[END_REF][START_REF] Makaraviciute | Development of a reusable protein G based SPR immunosensor for direct human growth hormone detection in real samples[END_REF][START_REF] Roche | Demonstration of a Reusable Plasmonic Polymer Microarray Sensing Platform[END_REF][START_REF] Zhao | A reusable localized surface plasmon resonance biosensor for quantitative detection of serum squamous cell carcinoma antigen in cervical cancer patients based on silver nanoparticles array[END_REF]. In one method, a used SPR sensor chip was treated chemically to remove the used receptors, and new receptor molecules were applied for addition sensing measurements [START_REF] Choi | Reusable biosensors via in situ electrochemical surface regeneration in microfluidic applications[END_REF][START_REF] Makaraviciute | Development of a reusable protein G based SPR immunosensor for direct human growth hormone detection in real samples[END_REF][START_REF] Zhao | A reusable localized surface plasmon resonance biosensor for quantitative detection of serum squamous cell carcinoma antigen in cervical cancer patients based on silver nanoparticles array[END_REF]. However, such a chemical treatment often contaminated sensor surfaces and degraded the sensor performance. Previous works show that chemical treatments can damage bioreceptors, causing losses of the sensor signals [START_REF] Goode | Biosensor regeneration: a review of common Techniques and outcomes[END_REF]. In another work, receptor-modified magnetic particles were assembled on a SPR sensor surface via external magnetic fields and used for SPR sensing measurements [START_REF] Lee | Surface plasmon resonance biosensing based on target-responsive mobility switch of magnetic nanoparticles under magnetic fields[END_REF]. After a sens ing, magnetic fields with an opposite direction were applied to remove the magnetic particles, leaving a fresh SPR sensor surface ready for additional sensing. This method based on magnetic fields can minimize the chemical contamination. However, the strong external magnetic fields may result in the aggregation of magnetic particles and leave residues on the SPR sensor surface, which may reduce the reliability of the sensor. Furthermore, in this method, an external magnetic field should be maintained during the SPR sensing process, and, thus, a conventional SPR sensing system should be modified to include an electromagnet device for sensing measurements. Until now, it is very difficult to build a SPR-based biosensor which can be reused repeatedly in a conventional SPR systems without degrading its signals. Another powerful strategy for cost-effective SPR biosensor can be optical fiber-based SPR sensors. The use of optical fibers for SPR sensing allowed one to develop low cost and even miniaturized SPR biosensors. However, such optical fiber-based SPR biosensors usually require specifically-configured equipment and cannot be utilized in conventional SPR systems [START_REF] Chiavaioli | Femtomolar detection by nanocoated fiber label-free biosensors[END_REF][START_REF] Guo | Fiber grating-assisted surface plasmon resonance for biochemical and electrochemical sensing[END_REF][START_REF] Loyez | Optical fiber gratings immunoassays[END_REF][START_REF] Shushama | Graphene coated fiber optic surface plasmon resonance biosensor for the DNA hybridization detection: simulation analysis[END_REF][START_REF] Zhao | Current status of optical fiber biosensor based on surface plasmon resonance[END_REF].

Herein, we report a reusable magnetic SPR sensor chip which allows us to repeatedly detect target analytes such as H1N1 virus in a conventional SPR system. In this strategy, ferromagnetic patterns on a SPR chip trapped a layer of magnetic particles with a fresh surface, and they were used as a solid substrate for SPR sensing in a conventional SPR system. Then, the used magnetic particles were removed via external magnetic fields, and a new layer of fresh magnetic particles was trapped on the SPR chip for additional sensing measurements. Since the ferromagnetic patterns were holding magnetic particles, our reusable SPR chip could be used for SPR sensing measurements in a conventional SPR system without any external magnetic fields. Furthermore, the strong external magnetic fields were deflected by the ferromagnetic patterns on the sensor surface, which avoided the large aggregation of magnetic particles on the surface and enabled rather large sensing signals comparable to those of conventional SPR chips. Using a single reusable SPR sensor chip, we could repeatedly detect the nucleoprotein (NP) of H1N1 influenza virus with the protein concentration ranging from 30 ng/ml to 10 μg/ml. Importantly, the sensing signals did not degrade even after more than 7 times of repeated SPR measurements. We also demonstrated the detection of different target molecules using a single SPR chip. Since the reusable SPR chip can be used repeatedly in a conventional SPR system and does not require any complicated chemical processes for refreshment, our method should dramatically reduce the cost for sensing processes and open up versatile practical applications of SPR sensors such as a molecular recognition study, a drug screening, a blood test, and a disease diagnosis.

Materials and methods

Materials

A superparamagnetic nanoparticle (nano-screenMAG-CMX) solution was purchased from Chemicell GmbH (Germany). The hydrodynamic size of a magnetic particle was about 100 nm, and the surface of the magnetic particle was covered with carboxymethyl-dextran (CMX) having a carboxyl group. 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC), N-Hydroxysuccinimide (NHS), and Ethanolamine hydrochloride-NaOH were purchased from GE Healthcare (Sweden). SIA KIT Au containing a SPR support, double-sided adhesive tape, and a protective sheath were also purchased from GE Healthcare. BioNano Health Guard Research Center (H-GUARD) prepared nucleoprotein (NP) of influenza type A H1N1 and its corresponding monoclonal antibody. 1x PBS (pH 7.4, 154 mM NaCl, 5.6 mM Na 2 HPO 4 , 1.1 mM KH 2 PO 4 ) was purchased from Lonza (USA).

Fabrication of nickel patterned reusable SPR sensor chip

4-inch glass wafers were purchased from Namkang Hi-Tech co. ltd (Korea). For the fabrication of a reusable SPR chip, the Cr/Au film (45 nm Au on 5 nm Cr) was first prepared on a glass wafer by a thermal evaporation process. Then, Ni/Au patterns (10 nm Au on 50 nm Ni, 5 μm × 10 μm) were fabricated by the photolithography and thermal evaporation.

Conventional SPR sensing using a reusable SPR sensor chip

The 0.1 mg/ml solution of protein G functionalized with cysteine was first flowed for 350 s. Since the cysteine groups formed sulfide or disulfide bonding with a gold surface, the protein G was attached to the gold surface of a sensor chip [START_REF] Lee | Direct immobilization of protein G variants with various numbers of cysteine residues on a gold surface[END_REF]. Afterwards, buffer solution (PBS) was flowed to remove the non-specifically bound protein G and stabilize a signal. The corresponding C-reactive protein (CRP) antibody solution of 0.1 mg/ml was flowed for 350 s to immobilize the antibody molecules on the protein G molecules. The buffer solution was flowed for 500 s to remove the non-specifically bound antibody molecules and stabilize a signal. Then, a CRP antigen solution of 5 μg/ml was flowed for 350 s, and the buffer solution was flowed. Here, the signal of CRP antigen is determined by a difference between the signal before and after the passage.

Repetitive trap and release of magnetic particles over a Ni patterned chip

First, the solution of fluorescence labeled magnetic particles was prepared. For trapping the magnetic particles on a Ni patterned chip, the solution was placed on the chip and a magnetic field of 150 mT was applied to the chip for 1 min. Then, it was washed lightly with PBS solution. For releasing the trapped magnetic particles, a magnetic field of 35 mT was applied for 30 s in the opposite direction to the previous magnetic field, while the chip was washed with DI water.

Repetitive sensing process using a reusable SPR sensor chip

A reusable SPR sensor chip was attached to a conventional SPR chip support by a piece of double-sided adhesive tape. 100 μl solution of magnetic particles (0.25 mg/ml) was introduced on the sensor chip, and an external magnetic field of 150 mT was applied for 1 min by a neodymium magnet. Then, the sensor chip was gently washed with PBS solution. The remaining solution was removed by a science wiper (Yuhan-Kimberly, model Kimtech Science Wiper). The sensor chip was installed on a protective sheath. Then, it was inserted to a conventional SPR detection system (Biacore T100, GE Healthcare). In the system, buffer solution (PBS) flow was first used for a few minutes to stabilize sensor signals. After stabilization of sensor signals, antibodies were immobilized on the magnetic particles following an amine coupling manual [START_REF] Fischer | Amine coupling through EDC/NHS: a practical approach[END_REF]. Mixed solution (0.1 M EDC and 0.4 M NHS) was introduced for 5 min to the sensor chip for activating the carboxyl group of magnetic particles. Then, antibody solution of 50 μl /ml was introduced for 5 min to the sensor chip. Within 5 min, SPR signals were saturated, indicating that antibody molecules were fully immobilized on the magnetic particles (Fig. S1 in Supplymentary Material). However, it is worth mentioning that some antibody molecules can be adsorbed in a wrong orientation due to the non-selective bindings of linker molecules and they may not function properly, which can be improved by utilizing other more selective linkers such as aptamers or affimers [START_REF] Centi | Aptamer-based detection of plasma proteins by an electrochemical assay coupled to magnetic beads[END_REF][START_REF] Li | On-chip, aptamer-based sandwich assay for detection of glycated hemoglobins via magnetic beads[END_REF][START_REF] Xie | Development of an Affimer-antibody combined immunological diagnosis kit for glypican-3[END_REF]. To block the remaining activated carboxyl groups of the magnetic particles, 1 M ethanolamine solution was injected to the sensor chip

U N C O R R E C T E D P R O O F
for 1 min. For the detection of target molecules, target molecules were introduced to the sensor chip for 300 s. Then, buffer solution was introduced to the sensor chip for 200 s to remove target molecules bound non-specifically to the sensor chip. We repeated the detection process with target solution having different concentrations of target molecules. After sensing, we took out the sensor chip from the SPR detection system. Then we applied a magnetic field of 35 mT in an opposite direction to the previous magnetic field. As a result, the used magnetic particles were removed from the chip surface. Then, the sensor chip was washed with DI water for 1 min and dried. In order to repeat the sensing process, the process described above was repeated.

Results and discussion

Repeated sensing processes of a reusable SPR sensor chip

Fig. 1 shows a schematic diagram depicting repeated sensing processes of a reusable SPR sensor chip. The detailed fabrication process of a reusable SPR chip is presented in the Materials and methods section. In brief, a metal film (45 nm Au on 5 nm Cr) was first deposited on a glass substrate. Then, Ni/Au pattern arrays (10 nm Au on 50 nm Ni, 5 μm × 10 μm) were fabricated on the substrate (Fig. 1A). Here, the Au layer was used to prevent the oxidation of Ni and to provide stable bindings of biochemical molecules for SPR sensing experiments. The substrate was attached to a sample holder of an SPR measurement system (Biacore, GE Healthcare) using a double-sided adhesive tape (center of Fig. 1). As a first step of sensing, the solution of magnetic particles functionalized with carboxylic groups was first introduced on the chip. Then, an external magnetic field (H-field, 150 mT) was applied on the substrate with perpendicular direction to the chip (Fig. 1B). Here, the external magnetic field magnetized the ferromagnetic Ni patterns (red arrow) and made magnetic fields near the patterns stronger than other regions. As a result, the magnetic particles were attracted toward the Ni patterns and trapped on them. The chip with trapped magnetic particles was washed with PBS to leave only the magnetic particles bound strongly on the magnetized Ni patterns. Note that the ferromagnetic Ni patterns can hold the trapped magnetic particles even without external magnetic fields. The substrate was inserted into a SPR detection system. In the SPR detection system, PBS solution flowed through a microfluidic channel to stabilize a sensor signal. Then, antibodies were immobilized on the magnetic particles by using EDC and NHS (Fig. 1C). After that, a target molecule solution was introduced for a specific sensing (Fig. 1D). In the sensing process, refractive index changes near the sensor chip surface were measured in a unit of RU (resonance unit). After the sensing, the sensor chip was taken out from the SPR detection system. For the removal of used magnetic particles, an external magnetic field with an opposite direction to the previous applied magnetic field was applied to the sensor chip (Fig. 1E). In this case, the external magnetic field was not strong enough to change the direction of magnetization of the Ni patterns. Thus, total magnetic field above the Ni patterns became weaker than those of other regions. Due to the magnetic field gradient, the magnetic particles were washed out from the sensor chip with DI water. As a result, we could recover a fresh surface of the sensor chip without magnetic particles for the next sensing measurements. In our method, magnetic fields were used to remove the used magnetic particles without any chemical treatment, which reduced chemical contamination on the SPR sensor surface. Furthermore, an external magnetic field doesn't have to be maintained during the SPR sensing process so that our reusable SPR sensor chip can be used in a conventional SPR sensing system.

Characterization of a SPR sensor chip with ferromagnetic Ni patterns

Fig. 2A shows the optical image of a reusable SPR sensor chip. A dark square region in Fig. 2A (i) is a ferromagnetic nickel pattern ar ray. Fig. 2A (ii) is the magnified image of Ni patterns. White squares are Ni patterns whose size are about 10 μm × 5 μm. These patterns were used for the trapping and releasing of magnetic particles. These results indicate that the sizes of Ni patterns and distances between them are uniformly controlled. The uniformly controlled Ni patterns evenly deflect an applied external magnetic field. As a result, they can hold magnetic particles without a large aggregation [START_REF] Yoo | Magnetically-refreshable receptor platform structures for reusable nano-biosensor chips[END_REF].

To confirm that the fabricated reusable SPR sensor chip shows similar SPR sensing signals as a conventional SPR sensor chip, we performed a conventional SPR sensing process using a reusable SPR sensor chip (Fig. 2B). The experimental procedure is in the Methods section. The SPR signal image shows increased SPR signals when protein G, C-reactive protein (CRP) antibody, and CRP antigen solutions were introduced on the sensor chip. These results indicate that the measured SPR signals from our reusable SPR sensor chip were similar to measured SPR signals from a conventional SPR sensor chip, even with a Ni pattern array fabricated on our reusable SPR sensor chip.

Magnetic trapping and releasing of magnetic particles with a reusable SPR sensor chip

Fig. 3A shows the simulation result of magnetic field distributions according to the applied magnetic fields at 1 mm above Ni patterns and the schematic diagrams of a simulation situation. The brighter regions represent those with a stronger magnetic field. Fig. 3A (i) shows a magnetic field distribution when a strong magnetic field was applied to ferromagnetic Ni patterns. The magnetic fields of the regions above the Ni patterns are stronger than those of the surrounding regions. Since magnetic fields caused by the magnetization of Ni patterns were added to the external magnetic field, the total magnetic fields near the patterns became relatively stronger than those of other regions. This magnetic field gradient generated a magnetic force that attracts paramagnetic materials such as magnetic particles. On the other hand, Fig. 3A (ii) shows a result when a rather weak magnetic field in the opposite direction to the previous applied magnetic field is applied to the Ni patterns. The magnetic field of the region above the Ni pattern is weaker than that of the surrounding portion. Since ferromagnetic materials have hysteresis characteristics for magnetization, they maintain the magnetization direction. Therefore, the magnetic field near the nickel pattern is reduced and becomes weaker than that of the surrounding area. This forces the magnetic particles on the nickel pattern to come out of the pattern. This simulation results show that we can control the trap and release of magnetic particles on ferromagnetic patterns via external magnetic fields [START_REF] Yoo | Magnetically-refreshable receptor platform structures for reusable nano-biosensor chips[END_REF].

Fig. 3B shows the fluorescence images of repeated trap and release of magnetic particles over a Ni patterned chip. The experimental procedure is in the Methods section. The above trap and release processes were repeated twice. The bright regions represent the trapped magnetic particles on Ni patterns. The above results show that the magnetic particles can be efficiently trapped and released repeatedly.

Fig. 3C shows SEM images taken after trapping magnetic particles on a Ni pattern (i), and releasing them (ii). The trapped magnetic particles are clearly shown in Fig. 3C (i) (marked by red arrows). Fig. 3C (i) shows that the magnetic particles were selectively trapped in the Ni pattern. Also, the magnetic particles were somewhat evenly distributed on the Ni pattern without large aggregations. As a result, our reusable SPR sensor chip with trapped magnetic particles can have optical properties similar to those of a bare Au film deposited on a glass substrate. It also should be mentioned that the coverage of nanoparticles on a single Ni pattern may not be uniform presumably due to some variations of individual patterns. However, since there are 375,000 of such Ni patterns on our sensor chip, we could still obtain uniform coverage of nanoparticles over the entire sensor surface [START_REF] Yoo | Magnetically-refreshable receptor platform structures for reusable nano-biosensor chips[END_REF]. On the other hand, in Fig. 3C (ii) the magnetic particles have clearly removed, indicating that our reusable SPR sensor chip can be reused for additional sensing processes.

Fig. 3D shows UV-VIS transmission spectra of an Au film, a Ni patterned Au film and a Ni patterned Au film with trapped magnetic particles. All the Au films were deposited on a glass substrate. Note that the transmittance intensity decreased after Ni patterning and magnetic particle trapping because the deposited Ni films and the trapped magnetic particles somewhat blocked lights [START_REF] Srivastava | Synthesis of superparamagnetic bare Fe3O4 nanostructures and core/shell (Fe3O4/ alginate) nanocomposites[END_REF][START_REF] Su | Nitride-based MSM UV photodetectors with photo-chemical annealing Schottky contacts[END_REF]. However, the peak wavelength of the transmissions remained as ~520 nm, which is similar to the previous results [START_REF] Axelevitch | Simulation and experimental investigation of optical transparency in gold island films[END_REF][START_REF] Siegel | Properties of gold nanostructures sputtered on glass[END_REF]. Also, the shape of the transmit tance curves did not change much even after the deposition of Ni layers and magnetic particles. These results show that the basic optical properties of Au film on a glass substrate were maintained even after Ni patterns were fabricated on Au films or magnetic particles were trapped on the Ni patterns on Au film. In this case, we can expect that SPR signals were not significantly affected because they are based on the change of evanescent waves rather than light transmission (Ekgasit et al., 2005). Therefore, for a sensing process using our reusable SPR sensor chip, we can obtain SPR signals similar to those of a conventional SPR sensing process.

Repeated SPR sensing measurements of NP and IL-13

Fig. 4A shows a real-time graph of a sensing process using a reusable SPR sensor chip. Target proteins were nucleoprotein (NP) of H1N1 viruses. The detailed experimental procedure is shown in the Materials and methods section. Here, after the stabilization of the sensor signal, the solution of NP with different concentrations from 300 ng/ml to 10 μg/ml was sequentially injected to the sensor chip. Note that the SPR signal immediately increases when the NP solution was injected to the sensor chip [START_REF] Chung | Application of SPR biosensor for medical diagnostics of human hepatitis B virus (hHBV). Sensors and Actuators B: Chemical[END_REF][START_REF] Nanduri | SPR biosensor for the detection of L-monocytogenes using phage-displayed antibody[END_REF]. Furthermore, the injection of the NP solution with a larger concentration resulted in a larger increase in the SPR signal. The slight decrease of SPR signals at a high antigen concentration is presumably due to the dissociation of NP and antibodies as reported previously [START_REF] Scarano | Surface plasmon resonance imaging for affinity-based biosensors[END_REF]. This result shows that our reusable SPR sensor chip could be used to measure the concentration of target molecules in real-time.

Fig. 4B shows SPR sensing results by 7 repeated sensing measurements using a single reusable SPR chip. The detailed experimental procedure is in the Methods section. A ΔSPR sensing signal was calculated from the difference of SPR signals before NP solution was injected Yoo et al. Biosensors and Bioelectronics xxx (xxxx) xxx-xxx to the SPR sensor chip. Note that, as the concentration of Np was increased, ΔSPR signals increased like a conventional SPR sensor chip [START_REF] Homola | Spectral surface plasmon resonance biosensor for detection of staphylococcal enterotoxin B in milk[END_REF][START_REF] Nanduri | SPR biosensor for the detection of L-monocytogenes using phage-displayed antibody[END_REF]. The average coefficient of variation (CV) was ~12% until the 5th repeated sensing measurements, which is in an acceptable range [START_REF] Jearanaikoon | The evaluation of loop-mediated isothermal amplification-quartz crystal microbalance (LAMP-QCM) biosensor as a real-time measurement of HPV16 DNA[END_REF][START_REF] Wang | Ultrasensitive colorimetric immunoassay for hCG detection based on dual catalysis of Au@Pt core-shell nanoparticle functionalized by horseradish peroxidase[END_REF]. In 6th and 7th measurements, the sensor exhibited some fluctuations at a rather low concentration condition with the total average CV of ~21%, presumably due to some contaminations on the substrate and small signals for low concentration samples. However, at a rather high concentration condition, we could still achieve high accuracy with 9% of average CV value for all 7 repeated sensing measurements. Fig. 4C shows the schematic diagram (i) and the sensing results (ii) of different target molecules using a single reusable sensor chip. Here, we repeated three different sensing measurements using a single SPR chip (Fig. 4C (i)). For the first measurement, the trapped magnetic particles were functionalized with Interleukin 13 (IL-13) antibody and IL-13 antigen solution ranging from 1 pg/ml to 30 ng/ml was sequentially measured. In the second measurement, NP solution of 10 μg/ml was measured without any magnetic particle and antibody. For the third measurement, NP solution ranging from 1 μg/ml to 10 μg/ ml was sequentially measured with NP antibody. After each measurement was completed, the used magnetic particles were removed via an external magnetic field for an additional sensing measurement. ΔSPR signal values at high concentration conditions. The results of the first measurement show that an increase in concentrations of IL-13 resulted in an increase of a normalized ΔSPR signal. By functionalizing the magnetic particles with the IL-13 antibody, we could successfully measure the IL-13 solution with our reusable SPR sensor chip. For the second measurement, the normalized ΔSPR signal is negligibly small. This result indicates that the SPR signal increases shown in Fig. 4A was caused by the specific reactions of NP molecules and their antibodies instead of non-specific bindings of NP molecules and Ni patterns. The results of the third measurement are similar to those of the measurements in Fig. 4B. Note that our reusable SPR sensor chip was used to measure two different target molecules (IL-13 in the first measurement and NP in the third measurement), which allowed one to overcome the limitation of a conventional SPR sensor chip. Additionally, although we conducted the first and second measurement before the third measurement, our reusable SPR sensor chip could be used to measure NP solution with reliable results. These results clearly show that our reusable sensor chip can be repeatedly used for sensing various target molecules without the degradation of signals.

U N C O R R E C T E D P R O O F H.

Conclusions

In conclusion, we developed a reusable SPR sensor chip which can be used to repeatedly detect different target molecules in a conventional SPR system. Here, we used ferromagnetic patterns fabricated on a SPR sensor chip to trap a layer of magnetic particles, and those with trapped magnetic particles were used as a substrate for a SPR sensing in a conventional SPR system. Note that the Ni patterns and trapped magnetic particles did not affect the SPR characteristics of our sensor chip. After the sensing was completed, we applied external magnetic fields to the patterns for removing the used magnetic particles. The magnetic particles were completely removed so that we could trap a new layer of fresh magnetic particles on the SPR chip for additional sensing measurements. With only a single reusable SPR sensor chip, we successfully repeated sensing measurements to detect the nucleoprotein (NP) of H1N1 influenza virus for 7 times with high accuracy. With only a single reusable SPR sensor chip, we successfully repeated sensing measurements to detect the nucleoprotein (NP) of H1N1 influenza virus for more than 7 times without significant degradation of sensing signals. Also, we demonstrated that a single SPR chip could be utilized to detect different target molecules. Our method is a powerful strategy which allows one to reuse rather expensive SPR chips in a conventional SPR system and bring them for versatile practical applications such as

Fig. 1 .

 1 Fig. 1. Schematic diagram depicting the cyclic process for the repeated sensing measurements using our reusable SPR biosensor chip. (A) A reusable SPR chip including ferromagnetic nickel patterns on a conventional SPR chip structure. (B)Trapping of magnetic particles on the reusable SPR chip through an external magnetic field. (C) Immobilization of antibodies on magnetic particles using EDC-NHS coupling in a conventional SPR system. (D) Detection of target molecules. (E) Removal of magnetic particles by an external magnetic field in an opposite direction to that for trapping.

Fig. 2 .

 2 Fig. 2. Characteristics of a reusable SPR sensor chip. (A) Optical image (i) of a reusable SPR sensor chip and magnified optical image (ii) showing Ni patterns. The patterns are uniformly fabricated. (B) Real-time sensing of CRP antigen using a reusable SPR sensor chip in a conventional SPR sensing process.

Fig. 3 .

 3 Fig. 3. Trapping and releasing of magnetic particles on reusable SPR sensor chips. (A) Simulation results showing magnetic field strength around Ni patterns during (i) trapping and (ii) releasing steps. (B) Fluorescence images of sequential trap and release of magnetic particles on Ni patterns. (C) SEM images after trapping magnetic particles (i) and releasing (ii) on a Ni pattern. (D) UV-vis spectra of Au film on glass substrate (black), Ni patterned Au film on glass substrate (blue), and magnetic particles trapped Ni patterned substrate (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

  Fig. 4C (ii) shows the normalized Δ SPR signals from our reusable SPR sensor chip during the repeated measurements of different target molecules with different concentrations. The normalized signals were calculated by normalizing Δ SPR signals with respect to their maximal

Fig. 4 .

 4 Fig. 4. Repeated SPR sensing measurements with reusable SPR sensor chips. (A) Real-time sensor responses after the sequential introduction of NP solution to a reusable SPR sensor chip. (B) Dose-dependence response of 7 repeated SPR sensing measurements with a single reusable SPR sensor chip. (C) Repeated SPR sensing measurement of two different target molecules (IL-13, NP) with a single reusable SPR sensor chip. (i) Schematic diagram of the repeated sensing measurement. (ii) Dose-dependence response of the measurement.

Acknowledgment

This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (MSIT) of Korea (No. 2013M3A6B2078961 and 2020R1A2B5B02002152), and the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 682286). S.H. also acknowledges the support from Samsung Electronics.

 Yoo et al. Biosensors and Bioelectronics xxx (xxxx)xxx-xxx a molecular recognition study, a drug screening, a blood test, and a disease diagnosis.

CRediT authorship contribution statement

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.bios.2020.112561.


Dr N Shin

Dr Y Cho

Prof J S Kim

Hong

Keywords: Au, scanning noise microscopy, semiconducting CNTs, temperature sensors, thin films

Semiconducting single-walled carbon nanotube-Au (SWCNT-Au) hybrid films are fabricated as a thermistor with an adjustable temperature coefficient and show that the embedded SWCNTs can dramatically reduce the resistivity and noise source activities in the film. The temperature coefficients of thin films are adjusted by the control of Au thicknesses. The scanning noise microscopy study shows that sSWCNTs-embedded Au film has reduced resistivity and noise source density by 3 and 18 times compared with a pristine Au thin film, respectively. The correlation between resistivity and noise source density indicates a hopping charge transport in the system. Q3 On the other hand, a key issue in using sSWCNT channels for device applications (i.e., a transistor, temperature sensors) can be the interface between sSWCNTs and metal electrodes. [10] Previous reports show that the formation of Schottky barriers between sSWCNT and metal electrodes may increase the contact resistance and often electrical noises, resulting in the degradation of overall device performances. [11,12] However, the effect of sSWCNTs on the nanoscale transport properties (i.e., conductivity, charge trap activities) of the metal-sSWCNT interface structures has not been clearly understood yet.

Herein, we report semiconducting SWCNTs embedded in Au films for a thermistor device with an adjustable temperature coefficient and reduced noise source activities. In this work, an Au thin film was deposited on sSWCNT network layers to build a thermistor device. Then, a scanning noise microscopy (SNM) method was utilized to map the resistivity and noise source activities on the hybrid film with a nanoscale resolution. The sSWCNT/ Au hybrid thin films showed a linear temperature-resistance dependence with minimal hysteresis. Interestingly, sSWCNTmetal hybrid films with rather thin Au films exhibited a negative temperature coefficient of resistance (TCR) values, while those with rather thick Au films showed a positive TCR. It indicates we can control the TCR values simply by changing the Au film thickness. The resistivity map measured by the SNM method exhibited three times smaller resistivity on the Au films with sSWCNTs, showing that sSWCNTs can work as a high conductance current path and enhance the conductivity of the film. Furthermore, we observed the 18 times smaller noise source activities in the region with sSWCNTs. These results show that sSWCNTs in thin metallic films can enhance the conductivity and reduce electrical noises, enabling high-performance device applications such as thermistors.

Results and Discussion

Figure 1a shows the fabrication process of an sSWCNT/Au hybrid thin film. First, photoresist (PR, Clariant Inc., AZ 5214-E) was patterned on a SiO 2 (300 nm)/Si substrate via a conventional photolithography method. The substrate was Semiconducting single-walled carbon nanotube-Au (sSWCNT-Au) hybrid films are prepared to build a thermistor device with an adjustable temperature coefficient and show that the embedded sSWCNTs reduce the resistivity and noise source activities in the nanoscale regions of the film. The temperature coefficient of the hybrid films can be adjusted from negative to positive values simply by increasing the deposited Au thickness. Moreover, a scanning noise microscopy is utilized to map the nanoscale variation of the resistivity and noise source density on the sSWCNT-Au hybrid films. Interestingly, the map shows that the regions with both sSWCNT and Au exhibit three times lower resistivity and 18 times smaller noise source activities than pristine Au regions, indicating that sSWCNTs embedded in metallic films can significantly reduce the resistivity and electrical noise source activities in the film.

Introduction

Semiconducting single-walled carbon nanotubes (sSWCNTs), due to their semiconducting properties and high conductivity, have been widely studied as a key component for various advanced devices such as a transistor, bioelectronic nose devices, and physical sensors. [1][2][3] One example of such advanced devices can be a sensitive temperature sensor with a positive-temperature coefficient (PTC). [4,[START_REF] Mohsin | Proc. SPIE 2014[END_REF] However, PTC sensors based on CNTs often have nonlinear resistance-temperature characteristics, which makes it very difficult to calibrate the sensor responses for practical applications. [START_REF] Andealis | [END_REF]4] The hybrid structures of CNTs and metal films also have been used to fabricate the temperature sensors. For example, CNT-Ni composites have rather linear resistance-temperature characteristics. [7] The temperature sensors based on CNTs coated with Pt nanoparticles exhibited a high sensitivity compared to pure Pt-based temperature sensors. [8,9] However, environmental conditions such as humidity can affect the responses of such sensors, often resulting in low reliability and repeatability of the sensors. placed in an sSWCNT (NanoIntegris Inc., purity ≈99.5% semiconducting) solution (0.05 mg mL -1 in 1,2-dichlorobenzene) for 5 h and rinsed with 1,2-dichlorobenzene. Then, Au layers with different thicknesses were deposited on the sSWCNTs via a thermal evaporation method with a deposition rate of 0.5 Å s -1 at a pressure of ≈10 -6 Torr. The PR patterns on the substrate were dissolved by acetone, leaving the pattern of an sSWCNT/ Au hybrid film. Finally, source and drain electrodes (30 nm Au on 10 nm Ti) were fabricated using photolithography processes including thermal evaporation and a lift-off method. The sample was utilized for SNM measurements. For a thermistor application, we deposited polymethyl methacrylate (PMMA) on the top of an sSWCNT/Au channel so that the PMMA could block the adsorption of moisture molecules affecting the electrical properties of the channel. [13] Figure 1b shows the experimental setup of our SNM measurement. [14] For the measurements, a Pt-based conducting tip (CONTSCPt, Park Systems) installed on an atomic force microscope (AFM, XE-70, Park Systems) was directly contacted to a bare sSWCNT/Au thin film surface without PMMA coatings. Then, a DC bias voltage was applied to the electrode of the sSWCNT/Au film by a function generator (DS345, Stanford Research Systems). The measured DC currents and AC current noise signals through the Pt tip were converted to voltage signals by a low-noise preamplifier (SR570, Stanford Research Systems). The probe was scanned on the sample surface to obtain the map of electrical currents and noises. The measured maps were analyzed by a network model-based differential method to obtain the maps of resistivity and noise source activities with a nanoscale resolution. [14] A Raman spectroscopy measurement (XperRam 200, 514 nm line from Ar + laser) was used to characterize the sSWCNT networks on a SiO 2 substrate (Figure 2a). Two major peaks at 1350 and 1600 cm -1 correspond to the D-band and the G-band peaks of sSWCNT networks, respectively. Note that the D-band originated from defects in sSWCNTs, while the G-band was associated with graphitic materials such as CNTs. [15,16] In our sample, the I D /I G value was ≈0.1, which was lower than that reported previously. [16] It shows low structural defects in our sSWCNTs deposited on a SiO 2 substrate.

Figure 2b shows the topographic image of an sSWCNT/Au film region with 5 nm thick Au (left) and a bare SiO 2 substrate (right). For the measurement, we used a noncontact mode AFM. In the figure, the thickness of the sSWCNT/Au hybrid film was rather uniform with its roughness of 1 nm. The measured diameters of sSWCNTs (white lines) were in the range of 1-2 nm. The result indicates that sSWCNTs in our sample were well-separated without forming significant bundles. [1,16] Figure 2c shows the I-V characteristic of an sSWCNT/Au hybrid film with the Au film thickness of ≈5 nm. A bias voltage ranged from -0.5 to 0.5 V was applied. We could obtain a nonlinear behavior on the sSWCNT/Au hybrid films, indicating the formation of Schottky barriers as reported previously. [10] Presumably, due to a very thin Au film, the sSWCNT/Au hybrid films still exhibited semiconducting properties like sSWCNTs, resulting in the formation of Schottky barriers between metal electrodes and sSWCNT channels.

U N C O R R E C T E D P R O O F
Figure 3a shows the optical image of a temperature sensor device based on an sSWCNT/Au hybrid film and the schematic diagram showing its structure. The sSWCNT/Au hybrid film channel has a dimension of 1 mm width and 1 cm length, and each electrode has a dimension of 7 mm width, 3 mm length, and 40 nm thickness. Further, the channel was covered by PMMA to prevent the effect of adsorbed moisture on the channel. [4] Figure 3b shows the real-time response of an sSWCNT/Au hybrid film with a 5 nm thick Au film by temperature changes. For the temperature control, the sSWCNT/Au thin film was directly placed on a heating stage, and the temperature of the heating stage was adjusted by a controller of the stage. A 0.1 V bias voltage was applied between source and drain electrodes using a semiconducting characterization system (Keithley, USA). The changes in current were measured while the temperature was adjusted between 30 and 40 °C, repeatedly. The result shows that the conductance of the sSWCNT/Au hybrid film increased or decreased as the temperature was increased or decreased, respectively, indicating the positive temperature coefficient. Presumably, since the deposited Au film was very thin, the hybrid film exhibited the semiconducting properties like sSWCNT networks. Importantly, we could obtain the similar variations of currents by three repeated temperature cycles, implying the high reliability of the hybrid films for a practical application. [17] Figure 3c shows the resistance variation of an sSWCNT/Au film with 5 nm thick Au under different temperature conditions. For the measurement, the temperature was swept repeatedly between 30 and 90 °C. The film exhibited the decreasing resistivity with increasing temperature with an almost-constant temperature coefficient of -3.3 × 10 -3 °C-1 over a rather wide range of temperature conditions. Importantly, the sensitivity of its resistance at different temperature conditions did not change even after repeated temperature sweeps, showing the repeatability of our results. Previous reports show that the temperature-dependent resistance curve of SWCNTs has a significant hysteresis behavior and it was attributed to the water molecules adsorbed onto the CNTs. [13] In our case, the PMMA layer blocks the adsorption of water, resulting in a minimal hysteresis, which is very important for practical applications of the films as a thermistor.

Figure 3d denotes the resistance variations of sSWCNT/Au films with different Au thicknesses by temperature changes. For preparing the samples, we assembled sSWCNTs on a SiO 2 substrate and deposited Au with different thicknesses ranging from 5 to 25 nm. Then, the resistance variations of each sample were measured as the temperature changes from 30 to 90 °C. The hybrid film with 5 nm thick Au and sSWCNTs exhibited a decreasing resistance with increasing temperature, indicating a negative temperature coefficient like pure sSWCNT networks. [18] Presumably, a 5 nm thick Au layer deposited on the sSWCNTs does not form a well-connected conduction channel, and the resistance of the sSWCNT/Au hybrid films is determined by that of sSWCNT networks. The negative temperature coefficient values for the hybrid films with less than 18 nm thick Au indicate the semiconducting properties of the film, showing that sSWCNT or sSWCNT-metal interface dominates its charge transport phenomena. As the thickness of Au increased, the temperature coefficient became more positive. In the case of hybrid films with 20 nm thick Au and sSWCNTs, its resistance increased with increasing temperatures like typical Au film channels. [19,20] The result can be attributed to the formation of continuous metallic Au film, dominating the charge transport of the hybrid films. The result indicates that we can fabricate a thermistor channel with the desired temperature coefficient simply by adjusting the Au thickness. It is also worth mentioning the advantage of utilizing purified semiconducting SWCNTs rather than unpurified mixed SWCNT materials including both metallic and semiconducting SWCNTs. The metallic and semiconducting SWCNTs have a positive and negative temperature coefficient, respectively. [21,22] Since it is often very difficult to control the composition of metallic and semiconducting SWCNTs in unpurified SWCNT materials, the temperature coefficient of unpurified SWCNT materials can vary significantly, resulting in a rather large uncontrollable variation of temperature coefficients in the hybrid films based on the unpurified SWCNTs. Thus, it is important to use semiconducting SWCNTs to achieve the hybrid films with a controllable temperature coefficient.

U N C O R R E C T E D P R O O F
The interface structures between semiconducting SWCNTs and metallic films have been extensively studied before. [10,19] However, the nanoscale effect of sSWCNTs on the charge transport of the interface is not fully understood yet. We utilized an SNM to map the nanoscale charge transport phenomena in the sSWCNT/metal hybrid films with 10 nm thick Au (Figure 4). We chose the film as a typical sample because it has a semiconducting property (Figure 3d), indicating that its charge transport properties are dominated by sSWCNT/metal interface structures rather than by thick metallic Au films. Figure 4a,b shows the AFM topography and current maps of a bare sSWCNT/Au thin film with a 10 nm thick Au layer, respectively. In the topography map, the sSWCNTs (white lines) are clearly distinguished from the pristine Au films (gray regions). In our sample preparation process, sSWCNTs were adsorbed on a bare SiO 2 substrate in the SWCNT solution, and the firstadsorbed layer of sSWCNTs on the SiO 2 substrate blocked the additional sSWCNT adsorption due to the "self-limiting" phenomenon. [23][24][25] Thus, we could prevent the formation of thick films with multiple sSWCNT layers and obtain almost a single layer of connected sSWCNT networks. The AFM image of our sSWCNT network film on a SiO 2 substrate shows bare SiO 2 regions as well as a connected network of sSWCNTs (Figure S1, Supporting Information). In this case, we could map both bare Au and sSWCNT-Au hybrid film regions in our SNM measurements on the sSWCNT-Au hybrid films. The measured heights of sSWCNTs varied between 1 and 2 nm, indicating that the sSWCNTs on our samples did not form significant bundles. [15,16] For mapping currents, a conducting AFM tip made direct contact and scanned on the sample surface while measuring electrical currents with a voltage of 5 mV on the electrode. In the current map, the regions with sSWCNTs (yellow lines) exhibited a larger current value than those without sSWCNTs (red regions), indicating sSWCNT/Au interface structures can work as a highly conducting current path.

We could place the conducting AFM probe at a desired location on the sSWCNT/Au sample surface and measure I-V characteristics from that location. Figure 4c shows the I-V characteristics which were measured at an sSWCNT/Au region (black line) and a pristine Au region (red line). Note that the I-V characteristic of the bare Au region exhibits a typical linear behavior as reported previously for metallic films. [19] On the other hand, the I-V characteristic of sSWCNT/Au regions exhibits a larger current value than a pristine Au region, which is consistent with the results in the current map (Figure 4b). However, at a rather large bias voltage, it shows a nonlinear behavior, indicating a Schottky barrier formation in the current path. Previous work showed that the Schottky barriers are formed between sSWCNTs and Au film and cause a nonlinear I-V curve, as we observed in our results. [10] For more quantitative analysis about the effect of sSWCNTs, we calculated a resistivity map from the current map using the iterative simulation method as reported previously (Figure 4d). [14,19,26] In the resistivity map, the regions with embedded sSWCNTs (yellow lines) can be easily identified as a low resistivity region, and they correspond to high current regions in the current map. The averaged resistivity of pure Au regions (blue regions) without sSWCNTs was 1.58 × 10 -5 Ω cm, which was similar to the previously reported resistivity of Au thin films. [27,28] On the other hand, sSWCNT/Au regions (yellow lines) exhibited the averaged resistivity value of 4.73 × 10 -6 Ω cm, which was much lower than that of the pristine Au regions. These results clearly show that sSWCNTs, even with the formation of Schottky barriers, work as a highly conducting current paths and reduce the overall resistivity of the hybrid films as reported before. [19,28] It should be mentioned that a rather low resistivity of our hybrid film can be advantageous for thermistor applications. Since the currents in a thermistor channel were often measured as a signal, a channel made of highly resistive The red or black line represents the data when the probe was in a pristine Au or sSWCNT/Au region, respectively. d) Resistivity map of the sSWCNT/Au sample. e) Current-normalized noise PSD dependence on frequencies. A 1/f-type noise behavior was observed. f) Current-normalized PSD map (at 17.3 Hz) on the film. h) Noise source density map of the sSWCNT/Au film. The pristine Au regions showed much higher noise source density than the sSWCNT/Au composite regions. h) Scatter plot showing a correlation between a noise source density Neff and resistivity. A positive correlation was observed with a scaling factor of 0.5. i) Histogram of resistivity with a log scale constructed from pixels in the resistivity map with different regions. It implies the log-normal distribution of resistivity in both pristine regions and sSWCNT/Au composite regions. j) Histogram of noise source density with a log scale constructed from pixels in the noise source density map. The noise source density showed a log-normal distribution in both pristine Au regions and sSWCNT composite regions. materials can provide only small signals and, often, result in a rather low signal-to-noise ratio. sSWCNTs have a significantly lower resistivity than conventional negative temperature coefficient materials such as transition metal oxide. For instance, our hybrid film with sSWCNTs and 10 nm thick Au film had a resistivity of 4.73 × 10 -6 Ω cm, while that of vanadium oxide, which has been extensively used in negative temperature coefficient thermistors, is usually larger than 10 -1 Ω cm. [29][30][31] Since our system included an FET network analyzer (SR770, Stanford Research Systems), we could place a conducting AFM probe on a specific location of our sample surface to measure the noise spectra. Figure 4e shows the graph of the current-normalized power spectral density (PSD) versus frequency f, which was measured at two different positions: i) Au region and ii) sSWCNT/Au region. The current-normalized PSD was measured in the frequency ranging from 3.9 Hz to 1 kHz with a bias voltage of 1 V. In the graph, two data obtained from Au and sSWCNT/Au regions show similar behavior. The scaling parameter of fitting lines is about -1 for both curves. A common theoretical model to explain low-frequency noises in thin Au films is the number fluctuation model, where individual noise sources trap and release charge carriers with a relaxation time τ. [32][33][34] In addition, previous works showed that electrical noises in CNT/Au thin films are mainly generated by number fluctuations. [33] Hence, the low-frequency noise of our hybrid sample could be attributed to the number fluctuation of charged carriers. For example, charge traps buried in underlying SiO 2 substrates can trap-and-release charge carriers resulting in current fluctuations. [14,19,[33][34][35] The repeated trap-and-release of charge carriers causes the PSD to vary in current proportional to ≈1/f 2 . If there are a large number of noise sources with different τ in the current channel like Au films, the noise PSDs of the different noise sources would be averaged out and result in the noise spectra is proportional to 1/f, which is consistent with our result.

Adv. Electron. Mater. 2020, 2000073 U N C O R R E C T E D P R O O F
The current-normalized PSD (S I /I 2 , Figure 4f) map could be obtained simultaneously while measuring the topography and current maps. Here, we first measured the map of noise power at 17.3 Hz via the SNM and divided the S I map by the square of the current map (Figure 4b) to obtain the noise PSD (S I /I 2 ) map. Note that the S I /I 2 values of sSWCNT/Au regions (black lines) were much lower than those of pure Au regions (white regions). Interestingly, the Au thin film regions with a higher resistivity in the resistivity map (Figure 4d) exhibited a larger noise PSD than sSWCNTs embedded regions in the noise power map (Figure 4e). The results imply that embedded sSWCNTs can reduce the electrical noises.

Electrical noises in a conducting channel can be generated by various noise sources. [26,33] In the case of sSWCNTs and metallic thin films on the SiO 2 substrate, localized charge traps in the underlying SiO 2 are reported as a major source of electrical noises. In this case, we can assume that each noise source traps and releases charge carriers independently with different relaxation times, and use a previously reported simulation method to obtain the map of the localized charge trap density N eff from the measured normalized PSD map (Figure 4g). [14,19,26] In the method, a differential analysis based on the Wiener-Khintchine theorem shows that the N eff in the small segment of a 2D film in the area of ΔxΔy can be written as

( , , ) ( ) ( , , ) eff 2 2
I N f x y c I f kT S f x y x y = ∆ × ∆ ∆ ∆ (1)
where the I, f, ΔC, and ΔS I are a current in the film segment, a frequency, a charge carrier number in the film segment, and a current noise PSD generated by the film segment, respectively.

For the calculation, we used the ΔxΔy value of ≈100 nm 2 which is the effective contact area of our conducting AFM probe provided by the manufacturer. For ΔC, we used a reported value (≈10 22 cm 3 ) of carrier concentrations in bulk Au. [36] The results show that the averaged N eff value of pure Au regions was about ≈5.8 × 10 17 cm -3 eV -1 (Figure 4g). The noise sources in the thin Au films can be attributed to the charge traps in the underlying SiO 2 substrate and some defects in the film. Previous work shows that the effective oxide trap density of general SiO 2 substrates was 10 16 -10 17 cm -3 eV -1 , which is consistent with our results. [37] On the other hand, the sSWCNT/Au regions could be easily identified as low noise source density regions (black lines) in the maps (Figure 4g). The averaged noise source density N eff of sSWCNT/Au regions was found ≈3.0 × 10 16 cm -3 eV -1 , which is 18 times lower than that of the pristine Au regions. Presumably, the sSWCNTs could reduce the N eff in the hybrid film by preventing electrons from being trapped in the underlying oxide substrates. The results clearly show that embedded sSWCNTs can reduce electrical noises as well as the resistivity in hybrid films.

Figure 4h shows a scattered plot of R s versus N eff . Thirty points on the plot were randomly taken from a pixel of the R s map (Figure 4d) and a corresponding pixel of the N eff map (Figure 4g). For clarity, the data points from each region were distinguished by different colors. The red or black data points represent the pixels of the image regions with or without sSWCNTs in the sSWCNT/Au hybrid film sample with 10 nm thick Au films, respectively. The positive correlation between the resistivity R s and the noise source activity N eff is observed with a slope of 0.5, indicating that the N eff can be a dominant parameter determining R s on our sSWCNT/Au hybrid thin films. When thin Au films are deposited on CNTs by a thermal evaporation method, Au has been reported to form discrete nanoparticles on the CNTs. [27,38,39] In such cases, the transport behavior of these CNT/metal composites has been reported to be dominated by hopping conduction. [40] The intrinsic hopping sites of CNT/metal composites are typically localized near the tails of band gaps, while trapping sites provided primarily by noises sources on SiO 2 substrates are extrinsic localized states, and large energy is required to release trapped carriers back to the intrinsic density of states (DOS). Thus, charge trapping and hopping sites are separated by the average trap depth (E t ), which can be an important parameter for determining the electrical characteristics of nanocomposites. In addition, the distribution widths of DOS of trapping sites (λ t ) and hopping sites (λ h ) can affect the transport properties in hopping conduction systems such as a nanocomposite. For example, it was previously reported that mobility (μ) was inversely proportional to the N eff , following the relationship like μ ∝ N eff ɤ where the exponent ɤ depends on E t , λ t , and λ h . [41] When traps are deep, and the λ h is smaller than λ t , the ɤ can be about ≈1 as formulated by Hoesterey Letson for a crystalline material. [42] On the other hand, when the ratio of λ t /λ h is about ≈1, the effective medium approximation theory suggested ɤ ∼ 0.5 for common nanocomposites. [41,43,44] Previous works showed that λ h /λ t is close to 1, and ɤ ∼ 0.5 in nanocomposite systems just like our conducting hybrid thin films, and thus, we can expect eff 0.5

N µ ∝ - (2)
in our cases. [41,43,45] Note that the R s can be written like

( ) s 1 R enµ ∝ - ( 3
)
where e is an electric charge and n is a constant carrier density. Thus, from Equations ( 2) and (3), we can derive the relationship of R s ∝ N eff 0.5 , which is consistent with our experimental observation in Figure 4h. This result indicates that charge traps can be one of the dominant parameters determining the conductivity of our sSWCNT/Au hybrid interface structures, which is also consistent with previous results. [14,19] Figure 4i,j shows the histograms of the R s and N eff with a log scale constructed from pixels in the R s (Figure 4c) and N eff (Figure 4f) maps, respectively. The logarithmic resistivity showed a normal distribution, i.e., the resistivity value is logarithmically distributed in both sSWCNT/Au and pure Au. It was previously reported that a log-normal distribution of resistivity comes from percolation characteristics. [1,46] The sSWCNT/ Au region could be considered as a percolated structure due to sSWCNT networks in the hybrid film, leading to the lognormally distributed resistivity. [46,47] The log-normal distribution of pure Au film regions in hybrid films could also be explained by the percolation structure of the Au thin film since very thin metallic films often exhibited percolative structures. [27,38] On the other hand, the N eff histogram on a log scale (Figure 4j) also shows two peaks of Gaussian distributions, which indicates that the histogram shows two overlapping log-normal distributions. These distributions can be explained by the calculation method for N eff as shown in Equation (1). The noise PSD of each pixel, ΔS I , in Equation (1) was just normally distributed due to the central limit theorem. [48] The distribution curve showed a narrow peak, implying a small standard deviation. On the other hand, the current of each pixel was lognormally distributed due to the lognormally distributed resistivity in percolating channels. Since the ΔS I values were rather uniformly distributed compared to the current values, the N eff distribution was dominated by the current distribution, resulting in the lognormal distributions.

Conclusion

In summary, we developed semiconducting SWCNT/Au hybrid films for a thermistor with an adjustable temperature coefficient and demonstrated the mapping of nanoscale transport phenomena in it. sSWCNT-metal hybrid films with rather thin Au exhibited a negative temperature coefficient like typical semiconducting materials, while positive temperature coefficient was observed for those with a rather thick Au film. The adjustable thermal coefficient can be a unique feature of sSWCNT/Au thin films compared with a conventional thermistor device and can enable versatile applications. Further, we mapped the resistivity and charge trap activities on the sSWCNT/Au thin films, showing that sSWCNTs embedded in metallic films can reduce its resistivity and noise sources. Our results show a key insight about the role of sSWCNTs in the interface between sSWCNT and metal electrodes, and thus provide an important guideline for the development of sSWCNT-based high-performance devices such as thermistors and thin-film transistors.

U N C O R R E C T E D P R O O F

N

  . Shin, Y. Cho, J. Kim, S. Hong* ................................... 2000073

Figure 1 .

 1 Figure 1. Schematic diagram depicting the fabrication method for sSWCNT/Au hybrid films, and the experimental setup for the mapping of electrical currents and noises on the film. A conductive probe installed on a contact-mode AFM was placed on the sSWCNT/Au surface, and a bias voltage was applied on the electrode at the other side of the Au surface.

Figure 2 .

 2 Figure 2. Basic characteristics of sSWCNT/Au thin films. a) Raman spectroscopy data of sSWCNTs used in our experiments. b) AFM topographic image of the sSWCNT/Au hybrid structure with 5 nm thick Au (left) and bare SiO 2 substrate (right). The bottom graph shows its height profile. c) I-V characteristic of the sSWCNT/Au with 5 nm thick Au.

Figure 3 .

 3 Figure 3. Characteristics of sSWCNT/Au film-based thermistor devices. a) Optical image and its schematic diagram of a thermistor based on sSWCNT/ Au hybrid film with 5 nm thick Au. b) Real-time response of a thermistor based on the sSWCNT/Au hybrid film with 5 nm thick Au. c) Temperature-dependent resistance changes of a thermistor based on sSWCNT/Au hybrid film with 5 nm thick Au. The measurement was repeated twice. d) Temperature-dependent resistance changes of thermistor devices based on sSWCNT/Au hybrid films with different Au thicknesses.

Figure 4 .

 4 Figure 4. SNM measurements on an sSWCNT/Au hybrid film with 10 nm thick Au. a) AFM topography image of the sSWCNT/Au film. b) Current map of the sSWCNT/Au film. c) I-V characteristics of the sSWCNT/Au film in different regions.The red or black line represents the data when the probe was in a pristine Au or sSWCNT/Au region, respectively. d) Resistivity map of the sSWCNT/Au sample. e) Current-normalized noise PSD dependence on frequencies. A 1/f-type noise behavior was observed. f) Current-normalized PSD map (at 17.3 Hz) on the film. h) Noise source density map of the sSWCNT/Au film. The pristine Au regions showed much higher noise source density than the sSWCNT/Au composite regions. h) Scatter plot showing a correlation between a noise source density Neff and resistivity. A positive correlation was observed with a scaling factor of 0.5. i) Histogram of resistivity with a log scale constructed from pixels in the resistivity map with different regions. It implies the log-normal distribution of resistivity in both pristine regions and sSWCNT/Au composite regions. j) Histogram of noise source density with a log scale constructed from pixels in the noise source density map. The noise source density showed a log-normal distribution in both pristine Au regions and sSWCNT composite regions.

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Carbon Nanotubes Embedded in a Metallic Film for a Thermistor Device with an Adjustable Temperature Coefficient and Reduced Noise Source Activities

	1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 Semiconducting Narae Shin, Youngtak Cho, Jeongsu Kim, and Seunghun Hong*

Q1 DOI: 10.1002/aelm.202000073

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Adv. Electron. Mater. 2020, 2000073

Acknowledgements

This work was supported by the Ministry of Science and ICT (MSIT) of Korea (no. 2013M3A6B2078961, 2020R1A2B5B02002152) and the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 682286). S.H. also acknowledges the support from the Samsung Electronics.

Supporting Information

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

Conflict of Interest

The authors declare no conflict of interest.