Surface Of Au Nanoparticles Research Articles

Gold nanoparticles supported on poly (aniline-co-pyrrole) as the efficient catalysts for the reduction of 4-nitrophenol

In this study, copolymer of poly (aniline-co-pyrrole) (P(ANI-co-Py)) was synthesized through copolymerization with (NH4)2S2O8 (APS) as oxidizing agent. Au nanoparticles (NPs) were deposited on P(ANI-co-Py) through the Au sol adsorption technique to obtain Au/P(ANI-co-Py) catalysts. For comparison, Au/PPy and Au/PANI were also prepared under the same procedure. The samples were characterized by SEM, TEM, XPS, N2 adsorption/desorption, XRD and FT-IR. Results imply that the P(ANI-co-Py) can be successfully prepared and can stabilize the small Au NPs with highly dispersed state. As expected, the Au/P(ANI-co-Py) catalysts, especially Au/P(5ANI-co-Py5), show efficient performance compared with Au/PANI and Au/PPy in 4-nitrophenol (4-NP) reduction. The outstanding catalytic activities of Au/P(5ANI-co-5Py) can be ascribed to well dispersed Au NPs, strong electronic metal-support interaction (EMSI), an appropriate adsorption of 4-NP and the negatively charged Au NPs surface. This work offers a facile method to provide conductive polymers supported Au catalysts with high catalytic performance and excellent reusability and clarifies their potential applications in industrial wastewater treatment.

Enhanced third-harmonic generation via gold nanospheres localized surface plasmonic resonance

In the progress of nanoengineering of noble metals, the extreme nonlinear optics within nanometric volumes is of great research interest in recent years. In particular, plasmonic nanostructures offer unique route for enhancing high-order harmonic generation and other nonlinear optical progress. Here, we report that strong third harmonic emission in zinc oxide (ZnO) microbelts (MBs) via a strong localization of electromagnetic field caused by localized surface plasmon resonances. In MB/Au-nanoparticles composite, it was demonstrated that the intensity of third-harmonic generation (THG) was enhanced by a factor of five compared to bare MB. Additionally, the polarization characteristics of THG in the same system were illustrated in detail. With respect to the bare MB, the polarization degree of THG was improved significantly. The nonlinear polarization features can be attributed to the crystal structure and the ensemble of anisotropic plasmonic hot spots on the surface of Au nanoparticles. Finally, intrinsic enhancement process of THG was explored comprehensively by finite difference time domain method. The simulation results are consistent with our experimental observation.

New Insights of Charge Transfer at Metal/Semiconductor Interfaces for Hot-Electron Generation Studied by Surface-Enhanced Raman Spectroscopy.

Plasmonic nanostructures with hot spots are very efficient in generating energetic (hot) electrons to realize light-driven chemical reactions. This effect primarily originates from high electric fields with nonuniform distribution in the hot-spot area. However, charge-transfer (CT) at plasmonic nanostructure interfaces and its effect on hot-electron generation have not been explored in detail. Here, a series of semiconductor/metal interfaces, with continuously adjustable energy-band structures, were constructed by the assembly of CdxZn1-xS supports and Au nanoparticles (NPs) interconnected with p-aminothiophenol (PATP) molecules. The plasmon-mediated oxidation of PATP embedded in CdxZn1-xS/PATP/45 nm-Au NP molecular junctions was systematically investigated using gap-mode-liked surface-enhanced Raman spectroscopy (SERS). Combining in situ SERS studies with energy-level analysis, interfacial CT was found to be a primary determinant of hot-electron-induced oxygen activation on large Au NP surfaces. This study provides a new perspective on the hot-electron generation mechanism to facilitate the rational design of efficient plasmonic photocatalysts.

Magnetization dynamics in the YIG/Au/YIG magnon valve

The magnetization dynamics of an yttrium iron garnet (YIG)/Au/YIG magnon valve was investigated using broadband ferromagnetic resonance. The material characterizations of YIG/Au/YIG were performed using cross-sectional scanning transmission electron microscopy, x-ray diffraction spectroscopy, x-ray photoemission spectroscopy, Raman spectroscopy, and UV–visible spectroscopy. Asymmetric Fano resonance in the YIG/Au (60 nm)/YIG magnon valve structure was observed experimentally, and the two coupled oscillators model was used to describe the source of the Fano resonance qualitatively. We also provide a quantitative description of the Fano resonance and extract the Fano factor, which is an important feature that can be used to define the interaction sign. This represents the first attempt to apply the Fano resonance to magnetization dynamics. The spin wave resonance modes excited by the Au nanoparticles (NPs) surface plasmons were also observed in a YIG/Au NPs/YIG structure. Our findings confirm the occurrence of magnetic Fano resonance in the YIG/Au/YIG magnon valve and pave the way toward the development of quantum information devices based on magnon valves.

WS2 coating and Au nanoparticle decoration of ZnO nanorods for improving light-activated NO2 sensing

ZnO-based heterogenous nanostructures are considered as excellent active materials for fabricating highly sensitive and selective gas sensors. In this work, nanocomposite WS2/Au/ZnO nanorods were synthesized by covering WS2 thin coatings on the surface of ZnO nanorods and embedding Au nanoparticles between the WS2 coatings and the ZnO nanorods, and the light activated sensing performance of the sensor using the WS2/Au/ZnO nanorods as the sensing material was evaluated. With an average ZnO nanorod diameter of 60 nm, WS2 coating thickness of 10 nm, and Au nanoparticle diameter of 5 nm, the WS2/Au/ZnO nanorods present high photoactivity because of widened photoresponse region, increased light absorption, and quenched photoluminescence. The performance for sensing NO2 gas of the WS2/Au/ZnO nanorod-based sensor under 365, 455 or 655 nm light activation was compared with that of a bare ZnO nanorod-based sensor. The WS2/Au/ZnO nanorod-based sensor exhibits better sensing performance than the bare ZnO nanorod-based sensor under 365 or 455 light activation, with the response to 20 ppm NO2 of the former 40% higher than that of the latter. The improved sensing performance is attributed to the co-sensitization of semiconductors and metals.

Extraordinary high preservation of the dispersion state of Au nanoparticles during freeze-thawing and freeze-drying with gum arabic

A series of edible and non-toxic substances were screened for their ability to prevent Au nanoparticles (AuNPs) from undergoing aggregation during both freeze-thawing and freeze-drying at as low an additive concentration as possible. Gum arabic was found to be an ideal choice. The addition of 20 µg/mL of gum arabic greatly inhibited the aggregation of AuNPs during freeze-thawing and -drying, whereas polyvinylpyrrolidone was the next best among the tested substances, with approximately 20% aggregation during freeze-drying at a concentration of less than 200 µg/mL. The zeta potential value of the AuNPs indicated the absence of the specific binding of gum arabic to the AuNPs surface. Surface enhanced infrared spectroscopic (SEIRAS) analyses were conducted for the gum arabic solution, frozen and freeze-dried on the Au particles. The results indicated that, in the frozen state, gum arabic molecules were concentrated in remaining unfrozen liquid on the Au coating without any specific orientation toward the Au surface. In contrast, SEIRAS spectra of freeze-dried gum arabic indicated that carboxylic groups in gum arabic molecules preferentially became closer to the Au surface. Considering these findings, during freeze-thawing, we conclude that the high thickening property of gum arabic gum plays an important role in minimizing the mobility and thus the aggregation of AuNPs in the freeze-concentrated regions between frozen ice crystals. The freeze-drying of a gum arabic solution produces a rigid amorphous matrix that has an affinity for the Au surface. The AuNPs were individually separated and immobilized in the freeze-dried matrix of the gum arabic, resulting in the extraordinary inhibition of their aggregation.

Probing catalytic reduction on the surface of Au nanoparticles by second-harmonic generation and two-photon luminescence.

The catalytic reduction of aromatic nitro compounds by metallic nanoparticles in the presence of sodium borohydride (NaBH4) has been widely studied as model reactions. However, the reaction mechanisms still need further investigations. For example, the origin of the induction time that has often been observed is still controversial. Here, we demonstrated that such catalytic reduction reactions on the surface of colloidal gold nanoparticles (AuNPs) may be inspected by the second-harmonic generation (SHG) and two-photon luminescence (TPL) emission from AuNPs. It was revealed that the SHG and TPL signals from AuNPs were sensitive to the substitution of citrate by active hydride species derived from the hydrolysis of NaBH4. Based on the UV-vis spectroscopy analyses and monitoring the SHG/TPL signals, the induction time in the catalytic reaction of 4-nitrothiophenol was revealed to originate from the hindered adsorption of hydride on the gold surface. This work demonstrated that SHG and TPL can provide a new approach for detecting active hydrides on the surface of metallic nanoparticles in colloids.

Dual functional catalysis of [Nb6O19]8--modified Au/Al2O3.

A catalyst prepared by modifying the surface of Au nanoparticles (NPs) on Al2O3 with [Nb6O19]8- clusters had specific base and reduction abilities, and the reduction of p-nitrophenol to p-aminophenol using H2 as a reductant proceeded efficiently with the dual functional catalyst. At the interface between Au NPs and basic [Nb6O19]8-, heterolytically cleaved hydrogen species are generated, which can efficiently react with nitrophenolate ions generated by base catalysis. Moreover, this surface modification strategy was applicable to the reduction of other nitro compounds.

The role of positively charge poly-L-lysine in the formation of high yield gold nanoplates on the surface for plasmonic sensing application.

An anisotropic structure, gold (Au) nanoplates was synthesized using a two-step wet chemical seed mediated growth method (SMGM) directly on the substrate surface. Prior to the synthesis process, poly-l-lysine (PLL) as a cation polymer was used to enhance the yield of grown Au nanoplates. The electrostatic interaction of positive charged by PLL with negative charges from citrate-capped gold nanoseeds contributes to the yield increment. The percentage of PLL was varied from 0% to 10% to study the morphology of Au nanoplates in term of shape, size and surface density. 5% PLL with single layer treatment produce a variety of plate shapes such as hexagonal, flat rod and triangular obtained over the whole substrate surface with the estimated maximum yield up to ca. 48%. The high yield of Au nanoplates exhibit dual plasmonic peaks response that are associated with transverse and longitudinal localized surface plasmon resonance (TSPR and LSPR). Then, the PLL treatment process was repeated twice resulting the increment of Au nanoplates products to ca. 60%. The thin film Au nanoplates was further used as sensing materials in plasmonic sensor for detection of boric acid. The anisotropic Au nanoplates have four sensing parameters being monitored when the medium changes, which are peak position (wavelength shift), intensity of TSPR and LSPR, and the changes on sensing responses. The sensor responses are based on the interaction of light with dielectric properties from surrounding medium. The resonance effect produces by a collection of electron vibration on the Au nanoparticles surface after hit by light are captured as the responses. As a conclusion, it was found that the PLL treatment is capable to promote high yield of Au nanoplates. Moreover, the high yield of the Au nanoplates is an indication as excellent candidate for sensing material in plasmonic sensor.

A Martini Coarse Grained Model of Citrate-Capped Gold Nanoparticles Interacting with Lipid Bilayers.

Citrate capping is one of the most common strategies to achieve the colloidal stability of Au nanoparticles (NPs) with diameters ranging from a few to hundreds of nanometers. Citrate-capped Au nanoparticles (CNPs) represent a step of the synthesis of Au NPs with specific functionalities, as CNPs can be further functionalized via ligand-exchange reactions, leading to the replacement of citrate with other organic ligands. In vitro, CNPs are also used to address the fundamental aspects of NP–membrane interactions, as they can directly interact with cells or model cell membranes. Their affinity for the bilayer is again mediated by the exchange of citrate with lipid molecules. Here, we propose a new computational model of CNPs compatible with the coarse grained Martini force field. The model, which we develop and validate through an extensive comparison with new all-atom molecular dynamics (MD) simulations and UV–vis and Fourier transform infrared spectroscopy data, is aimed at the MD simulation of the interaction between citrate-capped NPs and model phosphatidylcholine lipid membranes. As a test application we show that, during the interaction between a single CNP and a flat planar 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayer, the citrate coating is spontaneously replaced by lipids on the surface of Au NPs, while the NP size and shape determine the final structural configuration of the NP–bilayer complex.

Binary Surfactant-Mediated Tunable Nanotip Growth on Gold Nanoparticles and Applications in Photothermal Catalysis.

Plasmonic nanostructures with sharp tips are widely used for optical signal enhancement because of their strong light-confining abilities. These structures have a wide range of potential applications, for example, in sensing, bioimaging, and surface-enhanced Raman scattering. Au nanoparticles, which are important plasmonic materials with high photothermal conversion efficiencies in the visible to near-infrared region, have contributed greatly to the development of photothermal catalysis. However, the existing methods for synthesizing nanostructures with tips need the assistance of poly(vinylpyrrolidone), thiols, or biomolecules. This greatly hinders signal detection because of stubborn residues. Here, we propose an efficient binary surfactant–mediated method for controlling nanotip growth on Au nanoparticle surfaces. This avoids the effects of surfactants and can be used with other Au nanostructures. The Au architecture tip growth process can be controlled well by adjusting the ratio of hexadecyltrimethylammonium bromide to hexadecyltrimethylammonium chloride. This is due to the different levels of attraction between Br−/Cl− and Au3+ ions. The surface-enhanced Raman scattering and catalytic abilities of the synthesized nanoparticles with tips were evaluated by electromagnetic simulation and photothermal catalysis experiments (with 4-nitrothiophenol). The results show good potential for use in surface-enhanced Raman scattering applications. This method provides a new strategy for designing plasmonic photothermal nanostructures for chemical and biological applications.

Application of Alanine Scanning to Determination of Amino Acids Essential for Peptide Adsorption at the Solid/Solution Interface and Binding to the Receptor: Surface-Enhanced Raman/Infrared Spectroscopy versus Bioactivity Assays.

The article describes the application of the alanine-scanning technique used in combination with Raman, surface-enhanced Raman, attenuated total reflection Fourier transform infrared, and surface-enhanced infrared absorption (SEIRA) spectroscopies, which allowed defining the role of individual amino acid residues in the C-terminal 6–14 fragment of the bombesin chain (BN6–14) on the path of its adsorption on the surface of Ag (AgNPs) and Au nanoparticles (AuNPs). A reliable analysis of the SEIRA spectra of these peptides was possible, thanks to a curve fitting of these spectra. By combining alanine-scanning with biological activity studies using cell lines overexpressing bombesin receptors and the intracellular inositol monophosphate assay, it was possible to determine which peptide side chains play a significant role in binding a peptide to membrane-bound G protein-coupled receptors (GPCRs). Based on the analysis of spectral profiles and bioactivity results, conclusions for the specific peptide–metal and peptide–GPCR interactions were drawn and compared.

A Multifunctional Au/CeO2-Mg(OH)2 Catalyst for One-Pot Aerobic Oxidative Esterification of Aldehydes with Alcohols to Alkyl Esters.

Au nanoparticles bound to crystalline CeO2 nanograins that were dispersed on the nanoplate-like Mg(OH)2, denoted as Au/CeO2-Mg(OH)2, were developed as the highly active and selective multifunctional heterogeneous catalyst for direct oxidative esterification of aldehydes with alcohols to produce alkyl esters under base-free aerobic conditions using oxygen or air as the green oxidants. Au/CeO2-Mg(OH)2 converted 93.3% of methacrylaldehyde (MACR) to methyl methacrylate (MMA, monomer of poly(methyl methacrylate)) with 98.2% selectivity within 1 h, and was repeatedly used over eight recycle runs without regeneration. The catalyst was extensively applied to other aldehydes and alcohols to produce desirable alkyl esters. Comprehensive characterization analyses revealed that the strong metal–support interaction (SMSI) among the three catalytic components (Au, CeO2, and Mg(OH)2), and the proximity and strong contact between Au/CeO2 and the Mg(OH)2 surface were prominent factors that accelerated the reaction toward a desirable oxidative esterification pathway. During the reaction, MACR was adsorbed on the surface of CeO2-Mg(OH)2, upon which methanol was simultaneously activated for esterifying the adsorbed MACR. Hemiacetal-form intermediate species were subsequently produced and oxidized to MMA on the surface of the electron-rich Au nanoparticles bound to partially reduced CeO2−x with electron-donating properties. The present study provides new insights into the design of SMSI-induced supported-metal-nanoparticles for the development of novel, multifunctional, and heterogeneous catalysts.

Spectroscopically clean Au nanoparticles for catalytic decomposition of hydrogen peroxide

Au nanoparticles synthesized from colloidal techniques have the capability in many applications such as catalysis and sensing. Au nanoparticles function as both catalyst and highly sensitive SERS probe can be employed for sustainable and green catalytic process. However, capping ligands that are necessary to stabilize nanoparticles during synthesis are negative for catalytic activity. In this work, a simple effective mild thermal treatment to remove capping ligands meanwhile preserving the high SERS sensitivity of Au nanoparticles is reported. We show that under the optimal treatment conditions (250 °C for 2 h), 50 nm Au nanoparticles surfaces are free from any capping molecules. The catalytic activity of treated Au nanoparticles is studied through H2O2 decomposition, which proves that the treatment is favorable for catalytic performance improvement. A reaction intermediate during H2O2 decomposition is observed and identified.

Analyte-triggered citrate-stabilized Au nanoparticle aggregation with accelerated peroxidase-mimicking activity for catalysis-based colorimetric sensing of arsenite

Arsenite is a well-known toxic species widely distributed in many matrices, and exploring reliable and facile methods for its detection is always desired for environmental analysis and food safety control. Here we propose a catalytic colorimetric assay of arsenite based on the analyte inducing the aggregation of citrate-stabilized Au nanoparticles (AuNPs) with promoted peroxidase-mimicking activity. Originally, highly dispersed AuNPs with a mean size of ∼20 nm show negligible enzyme-like activity. However, when a small amount of arsenite is added, it triggers the rapid aggregation of AuNPs via the interactions between the analyte and the stabilizer citrate on AuNP surface, and it is further observed that the formed AuNP agglomerate can exhibit significantly accelerated peroxidase-mimetic activity to catalyze the 3,3′,5,5′-tetramethylbenzidine chromogenic reaction with the participation of H2O2. Based on this phenomenon, a facile colorimetric strategy with catalytic signal amplification was established for the efficient sensing of arsenite. With the catalysis-based strategy, arsenite in a wide range (0.0111.67 mg/L) was linearly determined with a detection limit lower than the standard approved by the US EPA and the EEA. Stable and reliable measurement of the analyte in various water matrices was also verified, promising our assay’s great potential in practical applications.

A facile and one-pot aqueous phase transfer of oleylamine capped Au NP with aminophenylboronic acid used as transfer and targeting ligand

Although various phase transfer techniques have been used to make hydrophobic nanoparticles (NPs) water-soluble. However, these techniques have been limited by inefficient surface modification strategy that often stable NPs in aqueous solutions. Herein, we report the use of 3-aminophenylboronic acid (3-APBA) as a hydrophilic ligand for phase transfer of oleylamine (OA) capped Au NPs (OA@Au NPs) from non-hydrolytic system into aqueous solutions. The 3-APBA capped Au NPs (3-APBA@Au NPs) was mainly characterized using different analytical techniques to substantiate the efficiency of the phase transfer procedure. In this simple procedure, 3-APBA molecule was simultaneously used as both phase transfer and targeting ligand for bacteria recognition in one step. In principle, while free electron pair of amin (:NH2) group of 3-APBAbind to surface of hydrophobic Au NPs for phase transfer, diol group can bind to glycan on the membrane of Staphylococcus aureus and Methicillin-resistant Staphylococcus aureus (MRSA) through proper cis-diol configuration. In addition, the resulting 3-APBA@Au NP can effectively catalyze the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of sodium borohydride (NaBH4) in aqueous solution.

Modulating the photoelectrochemical response of titanium dioxide (TiO2) photoelectrodes using gold (Au) nanoparticles excited at different wavelengths

Gold (Au) nanoparticles (NPs) have been widely used to modulate the photoelectrochemical current of TiO2 photoelectrodes in energy conversion and sensing systems. Understanding the different mechanisms responsible for photocurrent modulation at different frequencies is important for building optimized photoelectrochemical systems. Herein, we investigated the photocurrent magnitude at different excitation wavelengths by varying Au NP concentration at the photoelectrode surface. Under UV illumination, increasing the surface loading of Au NPs initially increased the photocurrent, and above a threshold loading level, decreased the measured photocurrent. However, under visible light excitation, increasing the Au NP surface density resulted in a steady increase in photocurrent. Mott-Schottky measurements, incident photon to current conversion efficiency measurements, and electrochemical impedance spectroscopy were used to understand the mechanisms responsible for these different observations. It was found that both current loss – due to reduced light absorption by TiO2 – and gain – due to direct charge transfer between Au and TiO2 NPs were possible under UV light. Under visible light illumination, strong light absorption and localized surface plasmon resonance of Au NPs and negligible light absorption by TiO2 NPs led to signal gain at varying Au NP surface concentrations. This bimodal signal modulation was further demonstrated in the context of biosensing using Au NP-labeled DNA barcodes, optically excited at different wavelengths. This study allows photoelectrochemical systems to be engineered for programmable signal-off, signal-on, or ratiometric biosensing combining the former sensing modes.

Luminescent properties of colloidal mixtures of Zn0.5Cd0.5S quantum dots and gold nanoparticles

The aim of the study is to establish spectral-luminescent interaction effects in mixtures of colloidal Zn0.5Cd0.5S quantum dots passivated with 2-mercaptopropionic acid and Au and Au/SiO2nanoparticles. The studied samples of Zn0.5Cd0.5S quantum dots, Au and Au/SiO2 nanoparticles and their mixtures were obtained by methods of colloidal synthesis and were characterised using transmission electron microscopy. The absorption, luminescence and time-resolved luminescence spectroscopy were used as the main investigation methods. The measurements were carried out at temperatures of 77 K and 300 K. The spectral-luminescent properties of “free” Zn0.5Cd0.5S quantum dots and those interacting with Au and Au/SiO2 nanoparticles were compared. It was found that the luminescence properties of Zn0.5Cd0.5S quantum dots can be controlled under conditions of changing plasmon-exciton coupling achieved during the formation of a dielectric SiO2 shell on the surface of Au nanoparticles as well as a result of a polymer introduced into the colloidal mixture.

Spectroscopic Evidence for a Covalent Sigma Au-C Bond on Au Surfaces Using 13C Isotope Labeling.

The Au–C linkage has been demonstrated as a robust interface for coupling thin organic films on Au surfaces. However, the nature of the Au–C interaction remains elusive up to now. Surface-enhanced Raman spectroscopy was previously used to assign a band at 412 cm–1 as a covalent sigma Au–C bond for films generated by spontaneous reduction of the 4-nitrobenzenediazonium salt on Au nanoparticles. However, this assignment is disputed based on our isotopic shift study. We now provide direct evidence for covalent Au–C bonds on the surface of Au nanoparticles using 13C cross-polarization/magic angle spinning solid-state NMR spectroscopy combined with isotope substitution. A 13C NMR shift at 165 ppm was identified as an aromatic carbon linked to the gold surface, while the shift at 148 ppm was attributed to C–C junctions in the arylated organic film. This demonstration of the covalent sigma Au–C bond fills the gap in metal–C bonds for organic films on surfaces, and it has great practical and theoretical significance in understanding and designing a molecular junction based on the Au–C bond.

2-thiazoline-2-thiol functionalized gold nanoparticles for detection of heavy metals, Hg(II) and Pb(II) and probing their competitive surface reactivity: A colorimetric, surface enhanced Raman scattering (SERS) and x-ray photoelectron spectroscopic (XPS) study

A novel 2-thiazoline-2-thiol functionalized gold (Au-TT) nanosensor was developed for the selective and sensitive detection of toxic heavy metal ions, Hg(II) and Pb(II) by colorimetry, and for studying its competitive surface reactivity using SERS and XPS. The detection mechanism and surface reactivity is based on competitive binding affinity of thiocarbonyl S or thiazoline ring N/S atom with the metal ions and the nanoparticles (NPs). Due to diff ;erences in the binding affinities of the metal ions towards the active binding sites of TT, addition of different metal ions resulted in variation of colour and SERS spectral features. Of the various metal ions studied, viz. Cu(II), Cd(II), Co(II), Zn(II), Ni(II), Hg(II), Pb(II), Ca(II), Fe(II), Mn(II) and Sn(II), only Hg(II) and Pb(II) showed distinctive colorimetric and SERS spectral response that was quantified using XPS. This study, thus, realizes a selective and sensitive visual based nanosensor for Hg(II) and Pb(II) with limit of detection (LOD) of ∼ 0.1 ppm. SERS revealed the formation of Hg(TT)2 and Pb(TT)2 complexes on the Au NPs surface with the former remaining bonded to the NPs, resulting in enhanced Raman intensity for the 1026 cm−1 band, while the latter gets desorbed from the surface leading to reduced SERS intensity.