Probing Biomolecular Interactions with Gold Nanoparticle-Decorated

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Probing Biomolecular Interactions with Gold Nanoparticle-Decorated Single-Walled Carbon Nanotubes Zachary P. Michael, Wen-Ting Shao, Dan C. Sorescu, Raymond Euler, Seth C. Burkert, and Alexander Star J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06056 • Publication Date (Web): 31 Aug 2017 Downloaded from http://pubs.acs.org on September 2, 2017

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The Journal of Physical Chemistry

Probing Biomolecular Interactions with Gold Nanoparticle-Decorated Single-Walled Carbon Nanotubes Zachary P. Michael†, Wenting Shao†, Dan C. Sorescu‡§, Raymond Euler†, Seth C. Burkert†, Alexander Star*† †

Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States ‡

United States Department of Energy, National Energy Technology Laboratory, Pittsburgh, Pennsylvania 15236, United States

§

Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States

ABSTRACT: Hybrid nanomaterials comprising metal-graphitic interfaces are uniquely suitable to probe molecular interactions and the associated phenomena such as charge transfer and adsorbate spillover effects. Herein, we study the modulation of the electronic and chemical properties of gold nanoparticledecorated single-walled carbon nanotubes (SWCNT) using Raman spectroscopy and measurements of field-effect transistor (FET) characteristics. SWCNT are extremely sensitive to changes in the local electronic environment and therefore gold-analyte interactions may be probed both through changes in FET characteristics (as an electrical transducer) and in surface-enhanced Raman scattering (as a chromophore). We study these changes both experimentally and theoretically in order to elucidate the electronic structure 1

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of complex nanocomposites, and the information gathered from these experiments is applied to the study of biomolecular interactions with gold nanoparticle-decorated SWCNT. This study, in addition to providing deeper understanding of metal-graphitic interfaces, will offer a combined approach to SWCNT biosensing methodology based on the dual monitoring of the FET-Raman characteristics which we demonstrate through detection of glutathione.

INTRODUCTION Hybrid materials based on single-walled carbon nanotubes (SWCNT), multi-walled carbon nanotubes (MWCNT), and graphene derivatives conjugated with metallic nanoparticles have been exploited for a broad range of sensing applications. Additionally, these graphitic materials have been used as substrates for surface-enhanced Raman scattering (SERS), where Raman signals are greatly enhanced in the immediate vicinity of plasmonic nanoparticles.1-7 The interface between metallic nanoparticles and graphitic surfaces is particularly interesting, and understanding this interface is instrumental in the design and implementation of new hybrid materials for catalysis, chemical sensing, and for a variety of other applications.8-11 Previously, we explored gold nanoparticle (AuNP) decorated carbon nanomaterials as chemical sensors1, 9 in addition to dual-mode measurements of biological systems utilizing nanotube fieldeffect transistor (FET) and spectroscopic characteristics.2 Chemical sensing with nanomaterial transducers continues to be an area of high interest due to their unique properties.12-13 Carbon nanomaterials, in particular, are widely studied because of their high aspect ratio, sensitivity to their local electronic environment, and their dimensions in the same size range as many important biomolecules.14-15 In addition, rich information may be gleaned from Raman spectra16-20 and transistor characteristics21 of these materials to further aid in the design of sensitive and specific probes for monitoring chemical and biological interactions. Metal-decorated semiconducting carbon nanomaterials offer a further advantage through the creation of “mini” Schottky barriers that contribute heavily to FETbased sensing mechanisms and act as strong charge scattering sites, resulting in highly sensitive functional 2


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hybrids for chemical sensing.22 Raman spectra for SWCNT are indicative of charge transfer events and may be probed with electrostatic17 and chemical23-25 modifications of their surface properties. Herein, we further probe the utility of Raman spectra and FET characteristics for applications in sensing, specifically using SWCNT as the chromophore in SERS spectra. We hypothesize that modifying the electronic properties of AuNP through chemical functionalization would lead to modulation of SWCNT Raman spectra, which can be used in conjunction with changes in FET characteristics to provide more indepth analysis of biosensing systems.

EXPERIMENTAL AND THEORETICAL METHODS Carbon nanotube preparation. Oxidized SWCNT (oxSWCNT) were prepared according to previously published work.26 Briefly, commercially available SWCNT (P2, Carbon Solutions, Inc.) were oxidized using a mixture of concentrated H2SO4:HNO3 (3:1) for two hours in a bath sonicator. These ox-SWCNT were filtered and rinsed thoroughly with water filtered through a Barnstead Thermolyne NANOpure DIamond ultrapure water system (nanopure water) to remove excess acid. The resulting material was suspended in nanopure water for further usage. Device fabrication and decoration with gold nanoparticles. Si wafers consisting of four interdigitated gold electrode devices were fabricated via a standard photolithography process (Figure 1a). These 2x2 mm2 chips were then wirebonded into standard 40-pin ceramic dual in-line packages (CerDIP) and secured with polydimethyl siloxane (PDMS) to protect and isolate the gold wires. OxSWCNT were then deposited between electrodes using AC dielectrophoretic (DEP) deposition (0.01 mg/mL oxSWCNT, 10 Vpeak-to-peak at 10 MHz for 60 s). Gold nanoparticles (AuNPs) were then introduced through bulk electrolysis in a 3electrode system (1 M Ag/AgCl reference electrode, Pt counter electrode, and oxSWCNT acting as the working electrode) from a AuCl3 solution (1 mM in 0.1 M HCl). The size of AuNPs was optimized for these experiments through control of the deposition voltage (-0.2 V) and time (30 s). 3



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FET measurements. FET characteristics of AuNPs decorated oxSWCNT (Au-SWCNT) FET devices (before and after biomolecule functionalization) were investigated using a liquid-gated FET device configuration. Two Keithley 2400 SourceMeters were employed for FET measurements. Phosphate buffered saline (PBS, 1 mM) was used as a gating medium and was added to a small fluid chamber (1 mL) placed over the FET devices. Characteristic FET curves (conductance (G) versus gate voltage (Vg)) were taken by sweeping gate voltage from +0.6 to -0.6 Vg (versus 1 M Ag/AgCl reference electrode in 1 mM PBS) with a fixed source-drain voltage at 50 mV. Functionalization of the FET devices with both reduced glutathione (GSH) and oxidized glutathione (GSSG) (Sigma Aldrich) was performed by incubating the FET devices in glutathione solutions in 10 mM PBS with concentrations ranging from 0.01 mM to 10 mM for GSH, and 0.005 mM to 5 mM for GSSG. Raman measurements. Biomolecule functionalized Au-SWCNT FET devices were fitted into a package holder and mounted onto a Horiba XploRA Plus confocal Raman microscope. Each Raman spectrum was recorded over 3 s with a 638 nm laser operating at 10% power. For GSH/GSSG study, 1 mM GSH and 0.5 mM GSSG were prepared in 10 mM PBS, and were introduced to two different Au-SWCNT FET devices respectively. Raman measurements were carried out every 10 minutes for a period of 1 h. Nanopure water was added to the FET device during the process to compensate for water evaporation. X-ray photoelectron spectroscopy measurements. High resolution X-ray photoelectron spectroscopy (XPS) spectra were collected on a Thermo ESCALAB 250Xi using monochromated Kα X-rays. The sample spot size was 500 μm with a pass energy of 50 eV. Charge compensation was provided by a low-energy electron source and Ar+ ions. Scanning electron microscopy imaging. Scanning electron microscopy (SEM) images were taken on a ZEISS Sigma 500 VP with an accelerating voltage of 10 kV. Computational methods. The adsorption properties of GSSG and GSH species on Au-SWCNT hybrid systems have been done using Vienna ab initio simulation package27-28 in conjunction to periodic slab 4


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models. Solution of the Kohn-Sham equations was obtained with a plane-wave basis set with a cutoff energy of 400 eV. The Perdew-Burke-Ernzerhof (PBE)29 exchange correlation functional corrected for long-range dispersion interactions using Grimme’s D3 method30 and the projector augmented wave (PAW) pseudopotentials31-32 were employed. In order to accommodate the relative large size of the GSSG and GSH molecules we used a (14,0) SWCNT with six repeating units (336 C atoms) along the tube axis. For AuNPs we used three different models with 20, 42 and 80 Au atoms. In all cases bonding of AuNPs to SWCNT takes place at a surface C-C defect functionalized with a mixture of COOH, O and H species. A vacuum width of 29 Å was used to ensure decoupling of neighbor slabs in the vertical direction, perpendicular on SWCNT axis. Given the large size of the supercell used (with dimensions of 25 x 25.01 x 40.0 Å3) sampling has been done using the Γ point during geometry optimization. In the case of density of states calculations the cutoff energy was increased to 500 eV and a finer 1x7x1 Monkhorst –Pack33 kpoint grid. For the adsorption configurations identified, the corresponding adsorption energies were determined based on expression Eads=(E(A)+E(S)-E(A+S)) where E(A) is the energy of the isolated adsorbate A, E(S) is the total energy of the relaxed slab, and E(A+S) is the energy of the combined adsorbate-slab system in the optimized configuration. Stable adsorption configurations correspond in this sign convention to positive adsorption energies. The amount of charge transferred between adsorbing molecule and AuNPs or between AuNPs and SWCNT were determined using the Bader set of charges.34 In this case the electronic charge density decomposition was obtained using the algorithm developed by Henkelman et al.35

RESULTS AND DISCUSSION 1. Raman measurements For all studies, Au-SWCNT were used. oxSWCNT were deposited using DEP on Si wafers between interdigitated source and drain gold electrodes (Figure 1a). AuNPs were then deposited from a solution of AuCl3 with bulk electrolysis in a three-electrode system, resulting in discrete AuNPs with size range 5



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between 10-100 nm anchored to the oxSWCNT (Figure 1b). Bonding of AuNPs to graphitic surfaces is thought to occur specifically at oxygenated defect sites.36 The presence of various oxygen-containing defects on oxSWCNT was detected using X-ray photoelectron spectroscopy (XPS, Figure S1). The O1s high resolution scan showed two peaks which correspond to C-O (531.71 eV) and C=O (533.01 eV) respectively, and the C1s high resolution scan indicates the presence of C-C (283.97 eV), C-O-C (285.00 eV), and O-C=O (288.40 eV), thus providing evidence for the presence of COOH, C=O, and OH functional groups on oxSWCNT. AuNPs may interact directly with the graphitic surface (through dispersion interactions), and shuttle freely along the surface until they encounter and anchor to oxygen-containing defect sites along the nanotube surface (Figure 1c). Such an effect can be seen from analysis of theoretical results based on density functional theory (DFT) calculations corrected to include long range dispersion interactions and used to describe the interaction between a Au20 cluster with a (14,0) SWCNT functionalized with oxygen-containing groups typically found in oxSWCNT (Figure S2). As seen from this figure, multiple oxygen-containing groups lead to an appreciable increase in adsorption of AuNPs on SWCNT relative to the case of the bare, pristine sp2 surface (panel a). This effect is valid for both non-vacancy defects (panels b-c) as well as for different types of vacancy defects functionalized with different combinations of COOH, O and H species (panels d-i), including the case of a Stone-Wales defect (panel j) functionalized using carboxyl groups.

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Figure 1. a) SEM micrograph of interdigitated gold electrode 4-device chips. b) SEM micrograph shows AuNPs attached to oxSWCNT. c) The DFT calculated structure of a Au80 cluster adsorbed at an oxygenated defect on a (14,0) SWCNT. d) FET characteristics of oxSWCNT before and after AuNPs electrodeposition. e) Raman spectra of oxSWCNT before and after AuNPs electrodeposition.

Nucleation of AuNPs on oxSWCNT was probed experimentally through liquid-gated FET measurements with a 1 M Ag/AgCl reference electrode as a gate electrode. Gate voltage (Vg) was swept from +0.6 to –0.6 V with 1 mM PBS as the electrolyte medium. As observed previously,2 upon AuNPs deposition there is an increase in conductance in oxSWCNT FET transfer characteristics (Figure 1d). Additionally, the Raman intensity of SWCNT increases by an order of magnitude due to SERS effects (Figure 1e).37 These results indicate strong coupling between AuNPs and oxSWCNT.

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We chose GSH and GSSG as probe molecules due to glutathione’s relatively simple tripeptide structure, the presence of the thiol-containing cysteine amino acid, and its anti-oxidant properties. Both GSH and GSSG adsorb on AuNPs through formation of Au-S bonds. GSH forms Au-S bonds by releasing H+, while for GSSG, formation of Au-S bonds involves incorporation of Au atoms into the S-S bond resulting in the formation of an S-Au-S motif. Consequently, to achieve the same coverage of GS- on Au-SWCNT at equilibrium for both glutathione species, concentrations of 1 mM GSH and 0.5 mM GSSG were used to study the interaction between GSH/GSSG and Au-SWCNT. By examining the Raman spectra before and after introduction of GSH and GSSG (Figures 2a), clear differences of the G peak, one of the characteristic SWCNT peaks, can be observed due to introduction of GSH and GSSG. The G peak, which is centered around 1590 cm-1, displays a feature around 1550 cm-1 after 1 h incubation with glutathione. Deconvolution of the G peak shows two peaks (i.e., G- and G+), where the G- peak corresponds to the metallic feature of SWCNT, and the G+ peak corresponds to the semiconducting feature. As shown by the deconvolution of the G peaks (Figure 2a insets), the G- peak exhibits an increase in intensity relative to the G+ peak, and width broadening after GSH or GSSG incubation. The transformation of semiconducting to metallic features in the SWCNT Raman spectra was previously attributed to molecular charge transfer.38 Comparisons of the G- peaks before and after GSH/GSSG incubation were plotted in Figure 2b to study the charge transfer effect by GSH/GSSG adsorption. All spectra were normalized to the G+ peak due to the lack of change in the G+ peak during the charge transfer process. The broadening and increase in intensity of the G- peak for both GSH and GSSG indicate an increase in SWCNT metallic feature, which is due to the charge-transfer between electron donor molecule and SWCNT.39 Therefore, Raman spectra suggested that, for both GSH and GSSG, adsorption onto SWCNT induced a charge transfer from the adsorbed glutathione species to the Au-SWCNT surface.

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Figure 2. a) Full Raman spectra of Au-SWCNT before (top) and after incubation with GSH (middle) and GSSG (bottom). The peaks labeled with asterisks are associated with silicon substrate. The insets are expanded views of the G band and indicate the deconvolution of the G band. b) G- peak before and after exposure to GSH (top) and to GSSG (bottom). The intensities of deconvoluted peaks were normalized to the G+ peak.

2. FET analysis For FET characteristics investigations, GSH and GSSG were dissolved in 10 mM PBS and introduced to Au-SWCNT FET devices for 1 h at each concentration, rinsed, and introduced to the next concentration. Characteristic FET curves were taken at each step in the process by sweeping the gate voltage from +0.6 to -0.6 Vg (versus 1 M Ag/AgCl reference electrode with 1 mM PBS as a gating medium) with a 50 mV bias 9



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voltage to acquire a baseline. All electrical measurements were conducted using two Keithley 2400 SourceMeters. Introduction of varying concentrations of GSH (0.01 to 10 mM) and GSSG (0.005 to 5 mM) to the Au-SWCNT FET devices results in consistent response in characteristic I-Vg curves (Figure 3a). The mechanism behind the consistent drop in device conductance is likely to be electron transfer into AuSWCNT, resulting in a decrease in the concentration of major carriers (holes), which in turn leads to a drop in conductance. Other effects, such as electron scattering effects, and electrostatic gating, can also contribute to the response for a typical SWCNT FET device.21

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Figure 3. a) FET characteristic curves before and after exposure to increasing concentrations of GSH and GSSG. Arrows indicate the direction of increasing concentration. b) The relative change in conductance at -0.6 Vg of Au-SWCNT FET devices to GSH and GSSG over a period of 1 h (red/blue dots). The data was fitted to a pseudo-second order kinetics equation (red/blue dashed line).

3. DFT results for the adsorption of GSSG and GSH molecules on Au-SWCNTs. Adsorption of GSSG or GSH molecules on Au-SWCNT is a facile process. We illustrate typical adsorption configurations for both these molecules and their corresponding relative energies in Figures S3 and S4. These results were obtained for the case of two different Au42 and Au80 nanoparticles adsorbed on an O-functionalized (14,0) SWCNT. In both cases, adsorption of GSSG and GSH species on AuNPs takes place through formation of covalent Au-S bonds and of additional long range dispersion interactions between the bulky groups of the molecules and AuNPs surface. Such a bonding scheme leads to very diverse binding configurations dependent both on the size and shape of AuNPs as well as on the individual conformer configuration of the GSSG or GSH molecules. As can be seen, adsorption can take place either at the corner or edges of small AuNPs (configurations S1 and S3 in Figure S3) or on the flatter portions of the larger Au80 nanoparticle (configurations S1 and S3 in Figure S4). In the case of GSH species, beside molecular adsorption with an intact molecule, dissociative adsorption of the SH group is thermodynamically favored. In this case, the proton is donated to the surface with formation of a stable Au-S-Au binding configuration (see panels S4 in Figures S3 and S4). Finally, the presence of an Au adatom near the adsorption site can lead to formation of other interesting binding motifs. For example, in the case of GSSG molecule, incorporation of the Au atom into the S-S bond can take place with formation of a SAu-S motif (see structures S2 in Figures S3 and S4). A similar motif can be formed in the case when two SH fragments are located in close proximity to the same Au adatom (see configurations S6 in Figure S3 and S5 in Figure S4). Overall, these findings illustrate that bonding of both GSSG and GSH species with AuNPs is highly favorable in either the molecular or dissociated forms and is mediated by formation of different Au-S-S-Au, Au-S-Au or Au-S-Au-S-Au motifs. 11



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4. Interpretation of the FET response for Au-SWCNT exposed to GSH and GSSG and correlations with DFT results. Similarity of the bonding motifs on gold surface can explain why the Au-SWCNT FET responses to the GSH and GSSG were very similar after one hour (Figure 3a). However, GSH and GSSG are expected to behave kinetically very differently due to the differences in steric hindrance between the monomer and dimer affecting chemisorption rates.40-41 A fixed concentration of GSH or GSSG (1 mM) was added to the device surface and FET measurements taken in the solution (t=0 being taken as the moment when the solution is first introduced). Transfer characteristics were taken in incremental time steps for 1 hour to examine the kinetics of GSH and GSSG reactions with Au-SWCNT FET devices. The response of the device over a period of 1 h was measured by calculating the relative change in conductance at -0.6 Vg for each time point during the experiment (Figure 3b, FET curves available in Figure S5). Due to the fact that adsorption of glutathione molecules is a chemisorption process, in which the rate-determining step is controlled by a chemical reaction (i.e., formation of Au-S bonds), FET results were fitted to a pseudosecond-order kinetics model42-43:

𝑅𝑅(𝑡𝑡) =

𝑘𝑘𝑅𝑅𝑒𝑒2 𝑡𝑡 1 + 𝑘𝑘𝑅𝑅𝑒𝑒 𝑡𝑡

where R(t) is the relative change in conductance (at -0.6 Vg) at time t, Re is the relative response at equilibrium, and k is the pseudo-second order constant (min-1). Using this expression, k was determined to be 0.719 min-1 and 0.298 min-1 for GSH and GSSG, respectively, with the response times, defined as the time required to reach 0.9 Re (or t90 response), of ~25 and ~79 min, respectively. The differences in k and t90 values represent an attractive method for distinguishing between the GSH and GSSG based on the response in Au-SWCNT FET devices.

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Molecular adsorption was found to take place with an important charge transfer from GSSG or GSH to AuNPs and further to the SWCNT. Pictorial views of the corresponding charge difference maps determined in these cases are presented in Figure 4 for both GSSG and GSH molecules in their undissociated, molecular states. For each case, we illustrate the data for both the molecule – Au interface (panels a and e in Figure 4) as well as for the Au-SWCNT interface (panels b and f in Figure 4). The analysis of the Bader charge distribution for each of the two molecules indicates a net charge transfer of 0.32 e and 0.37 e, respectively, from AuNPs to SWCNT. While the similarity in net charge transfer in the system between GSH and GSSG can explain the similarities in FET response curves, the differences in response of Raman spectra require more specialized computational analysis beyond the scope of this work.

Figure 4. Results of DFT calculations for two representative adsorption configurations of GSSG and GSH on the hybrid Au80-(14,0) SWCNT system. Panels (a) and (e) represent the charge difference maps for GSSG and GSH at the interface between respective molecules and Au nanocluster while panels (b) and (f) indicate the charge difference maps at the interface between Au nanocluster and SWCNT. In both cases the indicated isosurfaces have values of +0.012 e- /Å3 (yellow) and -0.012 e- /Å3(blue), respectively. Panels c) and g) indicate an isosurface of 0.01 e- /Å3 of the partial charge density corresponding to the electronic 13



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bands within a 0.2 eV interval above the Fermi level (taken as zero reference). Panels d) and h) represent the total density of states and the local density of states (LDOS) projections of SWCNT, the Au cluster and of individual GSSG or GSH molecules. The contributions of COOH, O and H atoms decorating the SWCNT have been included in the LDOS distribution of SWCNT.

A final aspect investigated theoretically is related to analysis of the electronic properties and their contribution to the sensitivity of Au-SWCNT to GSH and GSSG. We provide in panels d) and h) of Figure 4 the total density of states for the GSH (GSSG)-Au-SWCNT hybrid system together with the contributions of the localized densities of state (LDOS) components for the adsorbate molecule, AuNPs and SWCNT. As seen from both DOS curves, there is a non-zero electronic contribution above the Fermi level, indicative of a semimetal or metallic system. The nature of these states is illustrated in panels c) and g) of Figure 4 where we represent the partial charge density corresponding to the states within 0.2 eV above the Fermi level. As seen from both these panels, the charge density is delocalized on both the SWCNT and on the AuNPs and as such they are expected to contribute to the sensing performance of the Au-SWCNT hybrid system. GSH is a tripeptide which functions as an important antioxidant, and is typically found in a relative ratio to its disulfide oxidized form, GSSG. The relative ratio of these peptides is indicative of certain medical conditions including oxidative stress. As such, sensing and distinguishing the two forms of GSH is an important topic in the biomedical field. Au-SWCNT offer an ideal surface for studying GSH and GSSG due to the strong interaction of Au with thiols and disulfides. The combined FET-Raman method, where SWCNT are used as a sensitive chromophore for Raman and electrical transducer in FET devices, allows the detection of both GSH and GSSG.

CONCLUSIONS

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In conclusion, we discussed the effect of chemical modulation of Raman spectra of mixed networks of Au-SWCNT. These effects were utilized to examine the changes in characteristic Raman peaks as compared to changes in FET devices characteristics. By using SWCNT as a sensitive chromophore for Raman and electrical transducer in FET devices, we were able to produce dual mode sensors capable of studying –SH and S-S interactions with Au using an antioxidant biomolecule as a probe. Despite the inability to distinguish among GSH and GSSG due to their similar interactions with Au-SWCNT FET devices, it is possible to do so by adopting a dual monitoring of the changes in FET curves over a period of time and of the corresponding Raman characteristic peaks. We envision that such a combined FET-Raman method may have future applications in sensors and biosensors fields in general and in antioxidant capacity assays which rely on the oxidation of glutathione in particular.

ASSOCIATED CONTENT Supporting Information: Additional computational and experimental results. This material is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding author *Email: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT

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This work was partially supported by NSF CAREER award No. 0954435. The authors would like to thank Dr. Michelle M. Ward for usage of the University of Pittsburgh’s Undergraduate Analytical Laboratory Raman instrument.

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