Immobilization of green-synthesized silver nanoparticles for micro- and nano-spectroscopic applications: What is the role of used short amino- and thio-linkers and immobilization procedure on the SERS spectra?

Immobilization of nanoparticles (NPs) is a technique suitable for the preparation of large-scale substrates for surface-enhanced vibrational spectroscopy including micro- and nano-spectroscopic applications. The developed immobilization method provides the enhancing properties of the roughened substrate surface to be maintained for techniques like surface-enhanced Raman scattering (SERS) spectroscopy, however, at the same time the morphology is not limiting for related near-field (scanning probe) techniques. The study is focused on the comparison of different immobilization procedures of Ag nanoparticles and finding the relationship between preparation procedures leading to convenient surface morphology and the quality of the observed signal of the model analyte (riboflavin) using SERS. Amino-linker (3-aminopropyl)trimethoxysilane (APTMS) and four thio-linkers (cysteine, 3-mercaptopropanoic acid, 2-mercaptoethanol, and 2,2'-oxydiethanthiol) using five immobilization procedures at three different temperatures (23 °C, 40 °C, and 70 °C) were compared. Surface morphology was monitored by scanning electron microscopy and atomic force microscopy. The SERS spectra of riboflavin were evaluated in terms of the intensity and the resolution of individual bands. The spectral dataset was inspected by multivariate statistical methods - principal component analysis and discriminant analysis. The evaluation of spectra and statistical models show the influence of the used linker and AgNPs immobilization procedure on the spectral output. APTMS linker is less suitable; much more appropriate are thio-linkers deposited on an evaporated Au layer on a glass slide. The best spectral parameters were obtained for 2,2'-oxydiethanthiol and 23 °C.

Valence, Size, and Shape Control of Gold Nanoparticles Synthesized by Electron-Assisted Reduction

An electron-assisted strategy was developed to prepare gold nanoparticles (AuNPs) at room temperature. Glow discharge plasma as electron source was successfully used to control the valence state, size, and shape of AuNPs. Stable Au(I) was obtained in 3 min by plasma, and Au(I) was reduced to zero valence with the increase in treatment time. An increase in the amount of Au did not induce an increase in particle size. A narrow size distribution was also achieved. The narrowest size distribution was observed at 9 min at 600 V. AuNPs grew slowly under glow discharge plasma, which slightly changed the mean size of AuNPs. Moreover, the average size of AuNPs was smaller under alkaline conditions. The initial pH of the solution can affect the nucleation and growth of AuNPs and further affect their particle size. Spherical AuNPs, hexagonal AuNPs, rectangular AuNPs, flower-shaped AuNPs, and Au nanorods were easily obtained within 30 min by adding different additives. The hexagonal AuNPs exhibited the largest current response toward caffeine and showed a good linear range (0.1-1000 μM) with a low detection limit (0.064 μM), because their high-energy planes can increase the electron transfer rate and improve electrocatalytic activity.

Methods of Gold and Silver Nanoparticles Preparation

The versatile family of nanoparticles is considered to have a huge impact on the different fields of materials research, mostly nanoelectronics, catalytic chemistry and in study of cytocompatibility, targeted drug delivery and tissue engineering. Different approaches for nanoparticle preparation have been developed, not only based on "bottom up" and "top down" techniques, but also several procedures of effective nanoparticle modifications have been successfully used. This paper is focused on different techniques of nanoparticles' preparation, with primary focus on metal nanoparticles. Dispergation methods such as laser ablation and vacuum sputtering are introduced. Condensation methods such as reduction with sodium citrate, the Brust-Schiffrin method and approaches based on ultraviolet light or biosynthesis of silver and gold are also discussed. Basic properties of colloidal solutions are described. Also a historical overview of nanoparticles are briefly introduced together with short introduction to specific properties of nanoparticles and their solutions.

Ag Nanostructures with Spikes on Adhesive Tape as a Flexible Sers-Active Substrate for In Situ Trace Detection of Pesticides on Fruit Skin

Nanostructures with spikes (NSPs) have been a subject of several surface-enhanced Raman scattering (SERS) applications owing to their significant Raman signal enhancement brought about by the combined effects of interspike coupling and the accumulated induction on the tips of spikes. Thus, NSPs offer great potential as a SERS-active substrate for relevant applications that require a high density of enhanced “hot spots”. In this study, Ag NSPs were synthesized in varying degrees of agglomeration and were thereafter deposited onto a transparent adhesive tape as a flexible substrate for SERS applications, specifically, in the detection of trace amounts of pesticides. These flexible substrates were referred to as Ag NSPs/tape and optimized with an enhancement factor (EF) of ca. 1.7 × 10⁷. A strong resulting signal enhancement could be attributed to an optimal degree of agglomeration and, consequently, the distances among/between spikes. Long spikes on the synthesized core of Ag NSPs tend to be loosely spaced, which are suitable in detecting relatively large molecules that could access the spaces among the spikes where “hot spots” are generally formed. Since one side of the transparent tape is adhesive, the paste-and-peel off method was successful in obtaining phosmet and carbaryl residues from apple peels as reflected in the acquired SERS spectra. In situ trace detection of the pesticides at low concentrations down to 10⁻⁷ M could be demonstrated. In situ trace detection of mixed pesticides was possible as the characteristic peaks of both pesticides were observed in equimolar mixtures of the analytes at 10⁻² to 10⁻⁴ M. This study is, thus, premised upon applying for in situ trace detection on e.g., fruit skin.

Multifunctional paper strip based on GO-veiled Ag nanoparticles with highly SERS sensitive and deliverable properties for high-performance molecular detection

The development of paper-based SERS substrates that can allow multi-component detection in real-word scenarios is of great value for applications in molecule detection under complex conditions. Here, a multifunctional SERS-based paper sensing substrate has been developed through the uniform patterning of high-density arrays of GO-isolated Ag nanoparticles on the hydrophilic porous cellulose paper strip (GO@AgNP@paper). Wet-chemical synthesis was used to provide the cover of SERS hot spots on any part of the paper, not just limited surface deposition. In virtue of the inherent ability of paper to deliver analytes by the capillary force, the detection ability of the GO@AgNP@paper substrate was greatly promoted, allowing as low as 10^-19M R6G detection from microliter-volume (50 μL) samples. For the components with different polarity, the paper substrate can be used as an all-in-one machine to achieve the integration of separation and high-sensitive detection for ultralow mixture components, which improves the practical application value of SERS-based paper devices.