Uniform Surface-enhanced Raman Scattering Substrates Research Articles

Towards a scalable and controllable preparation of highly-uniform surface-enhanced Raman scattering substrates: Defect-free nanofilms as templates

The uniformity and reproducibility of substrates highly determine the applicability of surface-enhanced Raman scattering (SERS). Production of them, however, remains a challenge. Herein, we report a template-based strategy for the strictly controllable and handily scalable preparation of a very uniform SERS substrate, Ag nanoparticles (AgNPs)/nanofilm, where the template used is a flexible, transparent, self-standing, defect-free and robust nanofilm. Importantly, the obtained AgNPs/nanofilm is self-adhesive to surfaces of different properties and morphologies, ensuring in-situ and at real-time SERS detection. The enhancement factor (EF) of the substrate for rhodamine 6G (R6G) could reach 5.8 × 1010 with a detection limit (DL) of 1.0 × 10−15 mol L−1. Moreover, 500 bending tests and one-month storage showed no observable performance degradation, and up to 50.0 cm2 scaled-up preparation depicted negligible effect upon the structure and the sensing performance. The real-life applicability of AgNPs/nanofilm was demonstrated by the sensitive detection of tetramethylthiuram disulfide on cherry tomato and fentanyl in methanol with a routine handheld Raman spectrometer. This work thus provides a reliable strategy for large area wet-chemical preparation of high-quality SERS substrates.

Fast synthesis of gold nanostar SERS substrates based on ion-track etched membrane by one-step redox reaction

Surface-enhanced Raman scattering (SERS), due to its high detecting sensitivity and rapid data acquisition ability, has been considered as a powerful technique for label-free ultrasensitive detection of chemical and biochemical analytes. As an important part, the uniform SERS substrate is the prerequisite for this technology being used in all the related areas. Therefore, seeking the fast, convenient and low-cost way to obtain the SERS substrate with high performance and reproducibility never stops in recent decades. In this work, the PC membrane with uniform nanopores obtained by ion irradiation and chemical etching (i.e., ion-track etched PC membrane) was first used to prepare the gold nanostar SERS substrate. The monolayer gold nanostars can be obtained through a one-step redox reaction on the surface of the PC membrane, which not only can act as the base of SERS substrate but also can work as the reaction adjuster. By optimizing the growth conditions, the SERS substrate with uniform monolayer gold nanostars can be fabricated without any complicated procedures and costly equipment fast (in 20 mins). Meanwhile, the prepared flexible gold nanostar SERS substrate exhibits excellent Raman performance, which can effectively detect the analyte R6G with the concentration as low as 1 × 10-10 M and the SERS enhancement factors can be around 3.70 × 105. The new facile SERS substrate preparation method is cost-effective, convenient, fast and easily scale up, which can satisfy the requests of the real applications in many fields.

On-spot surface enhanced Raman scattering detection of Aflatoxin B1 in peanut extracts using gold nanobipyramids evenly trapped into the AAO nanoholes

Simple, rapid, convenient, and economical surface enhanced Raman scattering (SERS) substrate is developed for on-site evaluation of Aflatoxin B1 (AFB1) in food matrix using handheld Raman Spectrometer. Self-assembly of gold nanobipyramids (Au NBPs) into the nanoholes of anodic aluminum oxide (AAO) template/pattern using ‘drop-dry’ approach provides a reliable pathway for the rapid fabrication of highly active and uniform SERS substrate. It shows enhanced and reproducible SERS signals towards the probe molecule, 4-aminothiophenol (4-ATP) with a relative standard deviation (RSD) of less than 10% and an average enhancement factor (EF) of 1 × 108. For practical application, the proposed method is demonstrated for the detection of aflatoxin B1 (AFB1) in peanut extracts. The results show that the AFB1 in peanut extracts can be identified within 1 min, with a limit of detection of 0.5 μg/L. Compared with conventional ELISA based AFB1 analysis, our method is much more efficient (1 min versus >30 min).

Large-Scale Fabrication of Ultrasensitive and Uniform Surface-Enhanced Raman Scattering Substrates for the Trace Detection of Pesticides.

Technology transfer from laboratory into practical application needs to meet the demands of economic viability and operational simplicity. This paper reports a simple and convenient strategy to fabricate large-scale and ultrasensitive surface-enhanced Raman scattering (SERS) substrates. In this strategy, no toxic chemicals or sophisticated instruments are required to fabricate the SERS substrates. On one hand, Ag nanoparticles (NPs) with relatively uniform size were synthesized using the modified Tollens method, which employs an ultra-low concentration of Ag+ and excessive amounts of glucose as a reducing agent. On the other hand, when a drop of the colloidal Ag NPs dries on a horizontal solid surface, the droplet becomes ropy, turns into a layered structure under gravity, and hardens. During evaporation, capillary flow was burdened by viscidity resistance from the ropy glucose solution. Thus, the coffee-ring effect is eliminated, leading to a uniform deposition of Ag NPs. With this method, flat Ag NPs-based SERS active films were formed in array-well plates defined by hole-shaped polydimethylsiloxane (PDMS) structures bonded on glass substrates, which were made for convenient detection. The strong SERS activity of these substrates allowed us to reach detection limits down to 10−14 M of Rhodamine 6 G and 10−10 M of thiram (pesticide).

Silver nanoparticles deposited inverse opal film as a highly active and uniform SERS substrate

Ag-decorated TiO2 inverse opal films (ATIO) with high surface-enhanced Raman scattering (SERS) enhancement were prepared using an electroless deposition process. The Ag nanoparticles (NPs) are well-dispersed and deposited on the edge of macroporous walls. The structure and optical properties of the sample ATIO have been characterized. The Ag-loading cycles and pore sizes of TiO2 inverse opal are the key factors determining the magnitude of SERS signal enhancement. The optimized ATIO samples exhibit high SERS signal enhancement ability and reproducibility. The enhancement factor about 104 and the detection limit of 10−10M for R6G were achieved. The further application in detecting malachite green is demonstrated. A limit of detection approximately 10−9M was achieved. The results show that the ATIO nanostructure is promising for using as sensor substrates in SERS applications.

Nanoplasmonic Au nanodot arrays as an SERS substrate for biomedical applications

We demonstrate the preparation of gold nanodot arrays with nanoplasmonic structures for a surface enhanced Raman scattering (SERS) active substrate. The substrate was based on an anodized aluminum oxide template with various gold layer thicknesses. The activity of gold nanodot arrays as substrates was experimentally verified using Rhodamine B as an analyte. The experimental data showed increased enhancement as a function of Au layer thickness. The nanoplasmonic gold nanodot array structure presented in this study will allow for simple, low-cost fabrication of highly reproducible and uniform SERS substrate over a large area without the requirement for high resolution lithography. We demonstrated that gold nanodot arrays can be used as a platform for label-free RNA detection using SERS. This result could lead to the development of a nucleic acid diagnostic tool for biomedical applications using SERS detection.

Highly Reproducible Surface‐Enhanced Raman Spectra on Semiconductor SnO2 Octahedral Nanoparticles

Highly reproducible surface-enhanced Raman scattering (SERS) spectra are obtained on the surface of SnO(2) octahedral nanoparticles. The spot-to-spot SERS signals show a relative standard deviation (RSD) consistently below 20 % in the intensity of the main Raman peaks of 4-mercaptobenzoic acid (4-MBA) and 4-nitrobenzenethiol (4-NBT), indicating good spatial uniformity and reproducibility. The SERS signals are believed to mainly originate from a charge-transfer (CT) mechanism. Time-dependent density functional theory (TD-DFT) is used to simulate the SERS spectrum and interpret the chemical enhancement mechanism in the experiment. The research extends the application of SERS and also establishes a new uniform SERS substrate.

Microarrays of gold nanoparticle clusters fabricated by Stop&Go convective self-assembly for SERS-based sensor chips

SERS substrates fabricated from chemically synthesized nanoparticles (NPs) offer a distinct advantage of localizing and enhancing the electromagnetic fields by facile tuning of NP size, shape and interparticle distances. In this report, two-dimensional arrays of micrometre-sized clusters of gold nanoparticles protected by (i) sodium citrate and (ii) tris(2,4-dimethyl-5-sulfonatophenyl)phosphine (TDSP) ligands were directly assembled from colloidal suspensions onto flat, non-patterned substrates by discontinuous ('Stop&Go') convective self-assembly. The micrometric spacing between the NP clusters makes it easy to address them individually by confocal Raman microscopy. The packing of the gold NPs within these clusters with interparticle spacings of the order of nanometres leads to an optical response dominated by coupled surface plasmon resonances, and favours a strong enhancement of electromagnetic fields useful for surface enhanced Raman scattering (SERS). These NP clusters make very uniform SERS substrates, with reproducible SERS responses from cluster to cluster. The potential of these NP clusters for optical biosensing is demonstrated by the SERS detection of a biologically relevant molecule, cytosine, adsorbed onto the NP clusters. The presented results are promising for designing an original class of nanoparticle-based SERS microarrays. The new paradigm of convective self-assembly could be exploited generally for the patterning of various other types of colloidal micro- and nano-objects, such as semiconducting NPs, magnetic NPs, bacteria or proteins.

Silver‐decorated carbon nanotube networks as SERS substrates

AbstractWe report on investigations upon a surface‐enhanced Raman scattering (SERS) substrate produced from a two‐dimensional single‐walled carbon nanotube (SWNT) network decorated with Ag nanoparticles. Using the strong and unique Raman spectrum of SWNTs as a reference, the SWNT/Ag nanostructure can be considered to provide two regions: one with an ultrasensitive SERS response for single‐molecule SERS (SMSERS) study; and another with uniform SERS enhancement over an area of several square millimeters for general SERS measurements. We report the appearance of an anomalous Raman feature at around 2180 cm−1 in the high‐sensitivity region which exhibits the characteristics of SMSERS. The SERS performance of the uniform area was characterized using pyridine vapor adsorbed onto the substrate. The presence of the SWNT/Ag nanostructure enhanced the Raman intensity by over seven orders of magnitude, a factor comparable to or exceeding that obtained on SERS substrates reported by other groups. The results indicate great potential to produce highly sensitive, uniform SERS substrates via further fine‐tuning of the nanostructure. Copyright © 2011 John Wiley & Sons, Ltd.