Morphological and SERS Properties of Silver Nanorod Array Films Fabricated by Oblique Thermal Evaporation at Various Substrate Temperatures

Printable, stretchable metal-vapor-desorption layers for high-fidelity patterning in soft, freeform electronics

High-fidelity patterning of thin metal films on arbitrary soft substrates promises integrated circuits and devices that can significantly augment the morphological functionalities of freeform electronics. However, existing patterning methods that decisively rely on prefabricated rigid masks are severely incompatible with myriad surfaces. Here, we report printable, stretchable metal-vapor-desorption layers (s-MVDLs) that can enable high-fidelity patterning of thin metal films on freeform polymeric surfaces. The printed rubbery matrix with highly mobile chains effectively repels various metal vapors from the surface and inhibits their condensation, thereby allowing selective metal deposition. The s-MVDLs are printed by direct ink writing techniques, enabling customizable and scalable thin metal patterns ranging from the micrometer to millimeter scale with high fidelity. Furthermore, the superior stretchability and mechanical robustness of the s-MVDLs allow highly compliant deformation along the substrates, enabling the construction of unconventional circuits and devices on multi-curvature, non-developable, and stretchable surfaces.

Detection and Identification of Pesticides in Fruits Coupling to an Au-Au Nanorod Array SERS Substrate and RF-1D-CNN Model Analysis

In this research, a method was developed for fabricating Au-Au nanorod array substrates through the deposition of large-area Au nanostructures on an Au nanorod array using a galvanic cell reaction. The incorporation of a granular structure enhanced both the number and intensity of surface-enhanced Raman scattering (SERS) hot spots on the substrate, thereby elevating the SERS performance beyond that of substrates composed solely of an Au nanorod. Calculations using the finite difference time domain method confirmed the generation of a strong electromagnetic field around the nanoparticles. Motivated by the electromotive force, Au ions in the chloroauric acid solution were reduced to form nanostructures on the nanorod array. The size and distribution density of these granular nanostructures could be modulated by varying the reaction time and the concentration of chloroauric acid. The resulting Au-Au nanorod array substrate exhibited an active, uniform, and reproducible SERS effect. With 1,2-bis(4-pyridyl)ethylene as the probe molecule, the detection sensitivity of the Au-Au nanorod array substrate was enhanced to 10-11 M, improving by five orders of magnitude over the substrate consisting only of an Au nanorod array. For a practical application, this substrate was utilized for the detection of pesticides, including thiram, thiabendazole, carbendazim, and phosmet, within the concentration range of 10-4 to 5 × 10-7 M. An analytical model combining a random forest and a one-dimensional convolutional neural network, referring to the important variable-one-dimensional convolutional neural network model, was developed for the precise identification of thiram. This approach demonstrated significant potential for biochemical sensing and rapid on-site identification.

Unveiling practical considerations for reliable and standardized SERS measurements: lessons from a comprehensive review of oblique angle deposition-fabricated silver nanorod array substrates

Recently, there has been an exponential growth in the number of publications focusing on surface-enhanced Raman scattering (SERS), primarily driven by advancements in nanotechnology and the increasing demand for chemical and biological detection. While many of these publications have focused on the development of new substrates and detection-based applications, there is a noticeable lack of attention given to various practical issues related to SERS measurements and detection. This review aims to fill this gap by utilizing silver nanorod (AgNR) SERS substrates fabricated through the oblique angle deposition method as an illustrative example. The review highlights and addresses a range of practical issues associated with SERS measurements and detection. These include the optimization of SERS substrates in terms of morphology and structural design, considerations for measurement configurations such as polarization and the incident angle of the excitation laser, and exploration of enhancement mechanisms encompassing both intrinsic properties induced by the structure and materials, as well as extrinsic factors arising from wetting/dewetting phenomena and analyte size. The manufacturing and storage aspects of SERS substrates, including scalable fabrication techniques, contamination control, cleaning procedures, and appropriate storage methods, are also discussed. Furthermore, the review delves into device design considerations, such as well arrays, flow cells, and fiber probes, and explores various sample preparation methods such as drop-cast and immersion. Measurement issues, including the effect of excitation laser wavelength and power, as well as the influence of buffer, are thoroughly examined. Additionally, the review discusses spectral analysis techniques, encompassing baseline removal, chemometric analysis, and machine learning approaches. The wide range of AgNR-based applications of SERS, across various fields, is also explored. Throughout the comprehensive review, key lessons learned from collective findings are outlined and analyzed, particularly in the context of detailed SERS measurements and standardization. The review also provides insights into future challenges and perspectives in the field of SERS. It is our hope that this comprehensive review will serve as a valuable reference for researchers seeking to embark on in-depth studies and applications involving their own SERS substrates.

Reproducible SERS substrates manipulated by interparticle spacing and particle diameter of gold nano-island array using in-situ thermal evaporation

Gold (Au) nano-island arrays were deposited on the glass substrate to fabricate surface-enhanced Raman scattering (SERS) substrates by in-situ thermal evaporation (deposited and annealed samples at the same time). The optimal SERS intensity deposited by various thicknesses and in-situ annealing temperatures of Au nano-island arrays would be investigated. The biomolecules (adenine) were dropped on the well-designed SERS substrate for precise and quantitative SERS detection. The characterization of Au nano-island arrays SERS substrate would be evaluated by scanning electron microscope (SEM) and Raman spectroscopy. The results showed that the optimal deposition thickness and annealing temperature of Au nano-island arrays SERS substrate is about 14 nm and 200 °C respectively, which can construct the smallest interparticle spacing (W)/ particle diameter (D) ratio and the lowest reflection (%) and transmittance (%) to form the strongest SERS intensity. Moreover, finite-difference time-domain (FDTD) simulation of the electromagnetic field distributions on Au nano-island arrays displays the similar trend with the experimental results. The 14 nm deposition with 200 °C in-situ annealing temperature would display the highest density of hot-spots by FDTD simulation. The reproducible Au nano-island arrays SERS substrates with tunable surface roughness, W/D ratio, and lower reflection and transmittance show promising potential for SERS detection of biomolecules, bacteria, and viruses.

Self-Assembled Bifunctional Copper Hydroxide/Gold-Ordered Nanoarray Composites for Fast, Sensitive, and Recyclable SERS Detection of Hazardous Benzene Vapors

Volatile organic compounds (VOCs), particularly monoaromatic hydrocarbon compounds (MACHs), pose a potential risk to the atmospheric environment and human health. Therefore, the progressive development of efficient detection methodologies is a pertinent need, which is still a challenge at present. In this study, we present a rapid and sensitive method to detect trace amounts of MACHs using a bifunctional SERS composite substrate. We prepared an Au/SiO₂ enhanced layer and a porous Cu(OH)₂ adsorption layer via microfluidic-assisted gas-liquid interface self-assembly. The composite substrate effectively monitored changes in benzaldehyde using time-varying SERS spectra, and track-specifically identified various VOCs such as benzene, xylene, styrene, and nitrobenzene. In general, the substrate exhibited a rapid response time of 20 s to gaseous benzaldehyde, with a minimum detection concentration of less than 500 ppt. Further experimental assessments revealed an optimum Cu(OH)₂ thickness of the surrounding adsorption layer of 150 nm, which can achieve an efficient SERS response to MACHs. Furthermore, the recoverable and reusable property of the composite substrate highlights its practicality. This study presents a straightforward and efficient approach for detecting trace gaseous VOCs using SERS, with significant implications in the designing of SERS substrates for detecting other VOCs.

Raman spectra and DFT calculations of thiophenol molecules adsorbed on a gold surface

We report the calculation of Raman modes of thiophenol molecules adsorbed on a real gold surface. The calculated Raman spectra strongly depend on the absorption configuration of the molecule on the metallic surface, a feature that should be carefully taken into account in the interpretation of the surface enhanced Raman spectra (SERS). The calculated Raman spectra are compared with experimental SERS measurements, the best accordance being obtained for a tilted configuration of the absorbed molecule. The present study supports the necessary combination of computational approaches with SERS measurements to predict the type of molecular adsorption configurations on metallic surfaces.

GLAD Based Advanced Nanostructures for Diversified Biosensing Applications: Recent Progress

Glancing angle deposition (GLAD) is a technique for the fabrication of sculpted micro- and nanostructures under the conditions of oblique vapor flux incident and limited adatom diffusion. GLAD-based nanostructures are emerging platforms with broad sensing applications due to their high sensitivity, enhanced optical and catalytic properties, periodicity, and controlled morphology. GLAD-fabricated nanochips and substrates for chemical and biosensing applications are replacing conventionally used nanomaterials due to their broad scope, ease of fabrication, controlled growth parameters, and hence, sensing abilities. This review focuses on recent advances in the diverse nanostructures fabricated via GLAD and their applications in the biomedical field. The effects of morphology and deposition conditions on GLAD structures, their biosensing capability, and the use of these nanostructures for various biosensing applications such as surface plasmon resonance (SPR), fluorescence, surface-enhanced Raman spectroscopy (SERS), and colorimetric- and wettability-based bio-detection will be discussed in detail. GLAD has also found diverse applications in the case of molecular imaging techniques such as fluorescence, super-resolution, and photoacoustic imaging. In addition, some in vivo applications, such as drug delivery, have been discussed. Furthermore, we will also provide an overview of the status of GLAD technology as well as future challenges associated with GLAD-based nanostructures in the mentioned areas.

Preparation and SERS performance of silver nanowires arrays on paper by automatic writing method

Silver nanowire ink was written on the surface of drawing paper by automatic writing method. Scanning electron microscopy was used to characterize the surface morphologies of the drawing paper before and after writing silver nanowires. The effects of fabrication parameters and measurement parameters on silver nanowires arrays were investigated. Crystal violet was selected as the probe molecule to study the SERS performance of silver nanowires arrays. The detection limit of crystal violet was as low as 10^-15 mol/L. The uniformity and repeatability of the arrays were also explored, and the relative standard deviation values were about 10%. Moreover, silver nanowires arrays were also relatively stable that SERS signals were still observed after ten weeks. Detection of the crystal violet residue was further achieved on the substrates by continuously pressing nine times. In addition, silver nanowires arrays were also applied to the quantitative analyses of 2, 2'-bipyridyl.

Structure-Based Varieties of Polymeric Nanocarriers and Influences of Their Physicochemical Properties on Drug Delivery Profiles

Carriers are equally important as drugs. They can substantially improve bioavailability of cargos and safeguard healthy cells from toxic effects of certain therapeutics. Recently, polymeric nanocarriers (PNCs) have achieved significant success in delivering drugs not only to cells but also to subcellular organelles. Variety of natural sources, availability of different synthetic routes, versatile molecular architectures, exploitable physicochemical properties, biocompatibility, and biodegradability have presented polymers as one of the most desired materials for nanocarrier design. Recent innovative concepts and advances in PNC-associated nanotechnology are providing unprecedented opportunities to engineer nanocarriers and their functions. The efficiency of therapeutic loading has got considerably increased. Structural design-based varieties of PNCs are widely employed for the delivery of small therapeutic molecules to genes, and proteins. PNCs have gained ever-increasing attention and certainly paves the way to develop advanced nanomedicines. This article presents a comprehensive investigation of structural design-based varieties of PNCs and the influences of their physicochemical properties on drug delivery profiles with perspectives highlighting the inevitability of incorporating both the multi-stimuli-responsive and multi-drug delivery properties in a single carrier to design intelligent PNCs as new and emerging research directions in this rapidly developing area.

Surface Enhanced Raman Spectroscopy: Applications in Agriculture and Food Safety

Recent global warming has resulted in shifting of weather patterns and led to intensification of natural disasters and upsurges in pests and diseases. As a result, global food systems are under pressure and need adjustments to meet the change—often by pesticides. Unfortunately, such agrochemicals are harmful for humans and the environment, and consequently need to be monitored. Traditional detection methods currently used are time consuming in terms of sample preparation, are high cost, and devices are typically not portable. Recently, Surface Enhanced Raman Scattering (SERS) has emerged as an attractive candidate for rapid, high sensitivity and high selectivity detection of contaminants relevant to the food industry and environmental monitoring. In this review, the principles of SERS as well as recent SERS substrate fabrication methods are first discussed. Following this, their development and applications for agrifood safety is reviewed, with focus on detection of dye molecules, melamine in food products, and the detection of different classes of pesticides such as organophosphate and neonicotinoids.

Fabrication of Ag@Au (core@shell) nanorods as a SERS substrate by the oblique angle deposition process and sputtering technology

Gold (Au) and silver (Ag) are the main materials exhibiting strong Surface-Enhanced Raman Scattering (SERS) effects. The Ag nano-rods (AgNRs) and Au nano-rods (AuNRs) SERS substrates prepared using the technology of the oblique angle deposition (OAD) process have received considerable attention in recent years because of their rapid preparation process and good repeatability. However, AgNR substrates are unstable due to the low chemical stability of Ag. To overcome these limitations, an Ag@Au core-shell nano-rod (NR) array SERS substrate was fabricated using the OAD process and sputtering technology. Moreover, simulation analysis was performed using finite-difference time-domain calculations to evaluate the enhancement mechanism of the Ag@Au NR array substrate. Based on the simulation results and actual process conditions, the Ag@Au core-shell NR array substrate with the Au shell thickness of 20 nm was studied. To characterize the substrate's SERS performance, 1,2-bis(4-pyridyl)ethylene (BPE) was used as the Raman probe. The limit of detection of BPE could reach 10-12 M. The Ag@Au NR array substrate demonstrated uniformity with an acceptable relative standard deviation. Despite the strong oxidation of the hydrogen peroxide (H2O2) solution, the Ag@Au NR array substrate maintains good chemical stability and SERS performance. And long-term stability of the Ag@Au NR substrate was observed over 8 months of storage time. Our results show the successful preparation of a highly sensitive, repeatable and stable substrate. Furthermore, this substrate proves great potential in the field of biochemical sensing.

Recent advances in techniques for fabrication and characterization of nanogap biosensors: A review

Nanogap biosensors have fascinated researchers due to their excellent electrical properties. Nanogap biosensors comprise three arrays of electrodes that form nanometer-size gaps. The sensing gaps have become the major building blocks of several sensing applications, including bio- and chemosensors. One of the advantages of nanogap biosensors is that they can be fabricated in nanoscale size for various downstream applications. Several studies have been conducted on nanogap biosensors, and nanogap biosensors exhibit potential material properties. The possibilities of combining these unique properties with a nanoscale-gapped device and electrical detection systems allow excellent and potential prospects in biomolecular detection. However, their fabrication is challenging as the gap is becoming smaller. It includes high-cost, low-yield, and surface phenomena to move a step closer to the routine fabrications. This review summarizes different feasible techniques in the fabrication of nanogap electrodes, such as preparation by self-assembly with both conventional and nonconventional approaches. This review also presents a comprehensive analysis of the fabrication, potential applications, history, and the current status of nanogap biosensors with a special focus on nanogap-mediated bio- and chemical sonsors.

Biopolymer Composites with Ti/Au Nanostructures and Their Antibacterial Properties

In this study, we have aimed at the preparation and characterization of poly-l-lactic acid (PLLA) composites with antibacterial properties. Thin bilayers of titanium and gold of various thickness ratios were deposited on PLLA by a cathode sputtering method; selected samples were subsequently thermally treated. The surface morphology of the prepared composites was studied by atomic force, scanning electron, and laser confocal microscopy. The chemical properties of the composites were determined by X-ray photoelectron and energy-dispersive X-ray spectroscopy in combination with contact angle and zeta potential analyses. The antibacterial properties of selected samples were examined against a Gram-negative bacterial strain of E. coli. We have found that a certain combination of Au and Ti nanolayers in combination with heat treatment leads to the formation of a unique wrinkled pattern. Moreover, we have developed a simple technique by which a large-scale sample modification can be easily produced. The dimensions of wrinkles can be tailored by the sequence and thickness of the deposited metals. A selected combination of gold, titanium, and heat treatment led to the formation of a nanowrinkled pattern with excellent antibacterial properties.

SERS Activity of Silver Nanosphere, Triangular Nanoplates, Hexagonal Nanoplates and Quasi-Spherical Nanoparticles: Effect of Shape and Morphology

In this work, we prepared different morphologies of silver nanoparticles: nanosphere, triangular nanoplates, hexagonal nanoplates, and quasi-spherical shapes, through one-step synthesis. Hydrogen peroxide was used as the oxidizing agent during the reduction of silver nitrate by sodium borohydride, in the presence of tri-sodium citrate and poly-vinyl-pyrrolidone. The obtained silver nanoparticles were fully characterized by UV-Vis spectroscopy, Dynamic Light Scattering and Scanning Electron Microscopy, and successfully used as Surface Enhanced Raman Scattering (SERS) substrates. The effect of shape and morphology on the Raman scattering enhancement was evaluated by using methylene blue as target molecules. The Raman measurements demonstrated that the prepared substrates are reliable and sensitive with analytical enhancement factors, estimated to be around 105 with a concentration of methylene blue 1 μM. When triangular and hexagonal nanoplates were tested with different concentrations of analyte, they demonstrated a good linearity in Raman intensity with a good detection of methylene blue 0.1 μM.

Fabrication of Au Nanorods by the Oblique Angle Deposition Process for Trace Detection of Methamphetamine with Surface-Enhanced Raman Scattering

Oblique angle deposition (OAD) is a simple, low cost, effective, and maskless nanofabrication process. It can offer a reliable method for the mass fabrication of uniform metal nanorods which can be used as the surface-enhanced Raman scattering (SERS) substrate with an excellent enhancing performance. Up to now, Ag nanorods SERS substrates have been extensively studied. However, Ag is chemically active and easy to oxidize under atmospheric conditions. Comparatively, Au is chemically stable and has better biocompatibility than Ag. In this paper, we in detail, studied the electromechanical (EM) field distribution simulation, fabrication, and application of Au nanorods (AuNRs) on trace detection of methamphetamine. According to the finite-difference time-domain (FDTD) calculation results, the maximum EM intensity can be obtained with the length of AuNRs to be 800 nm and the tilting angle of AuNRs to be 71° respectively. The aligned Au nanorod array substrate was fabricated by the OAD process. The two key process parameters, deposition angle, and deposition rate were optimized by experiments, which were 86° and 2 Å/s, respectively. Using 1,2-bis (4-pyridyl) ethylene (BPE) as the probe molecule, the limit of detection (LOD) was characterized to be 10⁻¹¹ M. The AuNRs were also used to detect methamphetamine. The LOD can be down to M (i.e., 14.92 pg/ml), which meet the requirements of the on-site rapid detection of the methamphetamine in human urine (500 ng/ml).

Synthesis of high quality silver nanowires and their applications in ultrafast photonics

Silver nanowires are widely used in catalysts, surface enhanced Raman scattering, microelectronic equipment, thin film solar cells, microelectrodes and biosensors for their excellent conductivity, heat transfer, low surface resistance, high transparency and good biocompatibility. However, the optical nonlinearity of silver nanowires has not been further explored yet. In this paper, three silver nanowire samples with different concentrations are produced via a typical hydrothermal method. Their applications to fiber lasers are implemented to prove the optical nonlinearity of silver nanowires for the first time. Based on three kinds of silver nanowires, the mode-locked operation of fiber lasers is successfully realized. Moreover, the fiber laser based on the silver nanowire with a concentration of 2 mg/L demonstrates the shortest pulse duration of 149.3 fs. The experiment not only proves the optical nonlinearity of silver nanowires, but also has some enlightenment on the selection of the optimum concentration of silver nanowires in the consideration of ultrashort pulse output.

Effective Radiative Properties of Tilted Metallic Nanorod Arrays Considering Polarization Coupling

With the advent of new nanomanufacturing techniques has come the rise of the field of nanophotonics and an increased need to determine optical properties of novel structures. Commercial software packages are able to estimate the behavior, but require large resources and heavy computational time. By combining coordinate transforms and Effective Medium Theory (EMT), an effective relative permittivity tensor is defined and further exploited to calculate the polarization-coupled Fresnel coefficients through Maxwell’s equations. A uniaxial simplification is made to show the case of tilted nanorod arrays. To demonstrate the flexibility of this system, the interfacial reflectance has been calculated for both s- and p-polarizations as well as the coupled case with the volume filling fractions of f = 0.10 and 0.30 for silver (Ag) and titanium (Ti) nanorods, and a scenario of a Ag nanorod array with polymethyl methacrylate (PMMA) as the surrounding medium. The exact results computed by the finite-difference time-domain method justify the validity of EMT with polarization coupling taken into account. The effects of incidence angle and azimuthal angle on reflectance are also discussed. The relatively simple nature of this approach allows for fast estimations of the optical properties of various nanostructures.

Strong polarization-dependent terahertz modulation of aligned Ag nanowires on Si substrate

Optically tunable, strong polarization-dependent transmission of terahertz pulses through aligned Ag nanowires on a Si substrate is demonstrated. Terahertz pulses primarily pass through the Ag nanowires and the transmittance is weakly dependent on the angle between the direction of polarization of the terahertz pulse and the direction of nanowire alignment. However, the transmission of a terahertz pulse through optically excited materials strongly depends on the polarization direction. The extinction ratio increases as the power of the pumping laser increases. The enhanced polarization dependency is explained by the redistribution of photocarriers, which accelerates the sintering effect along the direction of alignment of the Ag nanowires. The photocarrier redistribution effect is examined by the enhancement of terahertz emission from the sample. Oblique metal nanowires on Si could be utilized for designing optically tunable terahertz polarization modulators.

Droplet-Confined Electroless Deposition of Silver Nanoparticles on Ordered Superhydrophobic Structures for High Uniform SERS Measurements

Surface-enhanced Raman scattering spectroscopy (SERS) is a nondestructive testing technique. To increase reproducibility of the SERS measurement is the key issue for improving the performance of SERS. In this article, we demonstrate an efficient method to improve the reproducibility, using confined silver nanoparticles (AgNPs) as a substrate. The AgNPs are formed uniformly on the tops of the prepared nanopillars by droplet-confined electroless deposition on the hydrophobic Si nanopillar arrays. The AgNPs present an excellent reproducibility in Raman measurement; the relative standard deviation is down to 3.40%. There exists a great linear correlation between the concentration of Rhodamine 6G (R6G) and the Raman intensity in the log-log plot; R² is 0.998, indicating that this SERS substrate can be applied for the quantitative SERS analysis. Meanwhile, the minimum detection concentration is down to 10^-11 M on the hydrophobic substrate, with R6G as a probe molecule.

Vertically standing nanoporous Al–Ag zig-zag silver nanorod arrays for highly active SERS substrates

We have demonstrated a simple de-alloying method to create nanogaps in a vertically standing zigzag AgNR arrays which act as SERS active hot spots for better SERS sensitivity.

Long-Standing Stability of Silver Nanorod Array Substrates Functionalized Using a Series of Thiols for a SERS-Based Sensing Application

Silver nanorod (AgNR) array substrates were fabricated using an oblique angle thermal evaporation technique; their long-term stability, surface uniformity and reproducibility, which are primary requirements for their widespread realistic application and commercialization, were assessed using surface-enhanced Raman scattering (SERS) spectroscopy. The nanorod surfaces were functionalized using a series of organic thiols, which range from hydrophilic to hydrophobic, to mimic various conditions that often arise during detection of hydrophilic/phobic analytes in a realistic application field. A group of these functionalized substrates was stored in ambient laboratory atmosphere; another in light minimized, moisture-free vacuum; while another was stowed carefully and neatly in water to mimic realistic conditions. The effects of these storing conditions were studied. A surfactant was added to the water to maintain consistent surface wetting in the third group. SERS spectra of nanorod substrates prior to functionalization were also recorded to investigate the effect of adventitious carbonaceous contaminants. A meticulous systematic study on the reproducibility of SERS signals was carried out: spot-to-spot, substrate-to-substrate, batch-to-batch, day-to-day. The relative standard deviation (RSD) shown by the SERS signals acquired from various spots of a single substrate was less than 3%, which is very similar to the only account reported so far, in which RSD is reported as 2%. The wetting behavior of these thiol functionalized AgNR substrates are investigated using static contact angle measurements. The functionalized substrates have exhibited excellent long-standing stability over a period of six months when stored appropriately; hence, they are highly suitable for mass production towards realistic application.

Assembly of Mid-Nanometer-Sized Gold Particles Capped with Mixed Alkanethiolate SAMs into High-Coverage Colloidal Films

We investigated the influence of the mixed n-alkanethiolate self-assembled monolayer (SAM) formed on gold nanoparticles (AuNPs: 50.0 ± 3.2 nm in diameter) on their assembly into colloidal films. Dodecanethiol and octadecanethiol were selected as the short- and long-chain alkanethiols, respectively. The mixed SAMs were formed by immersing AuNPs in a mixed alkanethiol solution at different molar ratios. Au colloidal films were fabricated on indium tin oxide substrates by our previously reported hybrid method. The composition of the two alkanethiolates in the SAM was deduced from the intensity ratio of two Raman bands at 1080 and 1105 cm(-1). The surface coverage of the colloidal films increased by forming equimolar or dodecanethiolate-dominant mixed SAMs on AuNPs instead of a pure dodecanethiolate or octadecanethiolate SAM. The highest coverage exceeded 80%. This improvement is attributed to the high dispersion stability of AuNPs covered with equimolar or dodecanethiolate-dominant mixed SAMs.