Plasmonic alloy nanochains assembled via dielectrophoresis for ultrasensitive SERS

Dielectrophoresis: Measurement technologies and auxiliary sensing applications

Dielectrophoresis (DEP), which arises from the interaction between dielectric particles and an aqueous solution in a nonuniform electric field, contributes to the manipulation of nano and microparticles in many fields, including colloid physics, analytical chemistry, molecular biology, clinical medicine, and pharmaceutics. The measurement of the DEP force could provide a more complete solution for verifying current classical DEP theories. This review reports various imaging, fluidic, optical, and mechanical approaches for measuring the DEP forces at different amplitudes and frequencies. The integration of DEP technology into sensors enables fast response, high sensitivity, precise discrimination, and label-free detection of proteins, bacteria, colloidal particles, and cells. Therefore, this review provides an in-depth overview of DEP-based fabrication and measurements. Depending on the measurement requirements, DEP manipulation can be classified into assistance and integration approaches to improve sensor performance. To this end, an overview is dedicated to developing the concept of trapping-on-sensing, improving its structure and performance, and realizing fully DEP-assisted lab-on-a-chip systems.

Electric field-induced assembly of Au-Ag alloy nanoparticles into nano-reticulation for ultrasensitive SERS

This paper discusses a method for assembling Au-Ag alloy nanoparticles (NPs) using direct current (DC) electric field to fabricate highly active SERS substrates. Different nanostructures could be obtained by regulating the intensity and action time of DC electric field. Under the condition of 5mA*10 min, we obtained Au-Ag alloy nano-reticulation (ANR) substrate with excellent SERS activity (Enhancement factor on order of magnitude of 10⁶). ANR substrate has excellent SERS performance due to the resonance matching between its LSPR mode and excitation wavelength. The uniformity of the Raman signal on ANR is greatly improved than bare ITO glass. ANR substrate also has the ability to detect multiple molecules: ANR substrate can respectively detect Rh6G and CV molecules with a concentration as low as 10^-10 M and 10^-9 M and the Raman spectral intensity of the probe molecules on the surface of the ANR substrate has good linear correlation with the molecular concentration (R²> 0.95). In addition, ANR substrate can detect both thiram and aspartame (APM) molecules far below (thiram for 0.0024 ppm and APM for 0.0625 g/L) the safety standard, which demonstrate its practical application potential.

Preparation of a Three-Dimensional Composite Structure Based on a Periodic Au@Ag Core-Shell Nanocube with Ultrasensitive Surface-Enhanced Raman Scattering for Rapid Detection

The absorption and scattering frequencies of surface plasmon resonance can be selectively adjusted by changing the morphology, size, structure, arrangement, and gap between noble metal nanoparticles so that the local electromagnetic field on the substrate surface can be further enhanced. This change will promote and popularize surface-enhanced Raman spectroscopy. This paper reports the research results and improvement scheme of surface enhanced Raman scattering (SERS) activity of silver-coated gold nanocubed/organism (Au@Ag/CW NCs) prepared by three-phase self-assembly. In the experiment, the uppermost oil phase in the three-phase self-assembly process was optimized as ethanol and n-hexane solution containing a specific concentration of a probe molecule rhodamine 6G or aspartame. The probe molecules were directly self-assembled on the surface of the composite substrate to avoid the possible loss and pollution during immersion and preservation and achieve the purpose of rapid detection. The results show that the Au@Ag/CW NC array substrate is a periodic cubic ring structure. The sensitivity, uniformity, reproducibility, and stability of composite Au@Ag/CW NC array substrates are verified by comparing the Raman activities of various substrates. The feasibility of using the substrate to realize rapid SERS detection, compared with the advantages and disadvantages of the traditional soaking method, proved that the prepared substrate and improvement direction have excellent potential for application and development prospects in the field of rapid food additive detection.

Turn-on stimuli-responsive switch: strategies for activating a new fluorescence channel by pressure

The stimulus-responsive smart switching of aggregation-induced emission (AIE) features has attracted considerable attention in 4D information encryption, optical sensors and biological imaging. Nevertheless, for some AIE-inactive triphenylamine (TPA) derivatives, activating the fluorescence channel of TPA remains a challenge based on their intrinsic molecular configuration. Here, we took a new design strategy for opening a new fluorescence channel and enhancing AIE efficiency for (E)-1-(((4-(diphenylamino)phenyl)imino)methyl)naphthalen-2-ol. The turn-on methodology employed is based on pressure induction. Combining ultrafast and Raman spectra with high-pressure in situ showed that activating the new fluorescence channel stemmed from restraining intramolecular twist rotation. Twisted intramolecular charge transfer (TICT) and intramolecular vibration were restricted, which induced an increase in AIE efficiency. This approach provides a new strategy for the development of stimulus-responsive smart-switch materials.

Electrically controllable self-assembly of gold nanorods into a plasmonic nanostructure for highly efficiency SERS

In this Letter, a method for the rapid and efficient preparation of ultrasensitive detection substrates by assembling gold nanorod suspensions with the application of an alternating current (AC) field is proposed, and it is found that frequency and voltage are the effective means of regulation. A sandwich structure (parallel SiO₂ plate) not only effectively slows down the evaporation rate, but also visually reveals the changes in the assembly process. Under the optimal assembly conditions, the sensitivity and uniformity of the substrate to different probe molecules are tested. The Raman detection results experimentally show that the detection limits of Rhodamine 6G (Rh6G), crystal violet (CV), and Aspartame (APM) molecular solutions are 10^-14 M, 10^-10 M, and 62.5 mg/L, respectively, and the mixed dye molecular solutions can also be effectively distinguished. Furthermore, Rh6G and CV characteristic peaks at 1647 cm^-1 and 1619 cm^-1 were measured at randomly selected positions, and their relative standard deviations (RSDs) were 5.63% and 8.45%, respectively, indicating that the substrate has good uniformity. The effective regulation of the self-assembly results of nanoparticles will further enhance the practical application effect of surface-enhanced Raman technology and expand the application prospects of this technology.

Applications of surface-enhanced Raman spectroscopy in environmental detection

As the human population grows, the anthropogenic impacts from various agricultural and industrial processes produce unwanted contaminants in the environment. The accurate, sensitive and rapid detection of such contaminants is vital for human health and safety. Surface-enhanced Raman spectroscopy (SERS) is a valuable analytical tool with wide applications in environmental contaminant monitoring. The aim of this review is to summarize recent advancements within SERS research as it applies to environmental detection, with a focus on research published or accessible from January 2021 through December 2021 including early-access publications. Our goal is to provide a wide breadth of information that can be used to provide background knowledge of the field, as well as inform and encourage further development of SERS techniques in protecting environmental quality and safety. Specifically, we highlight the characteristics of effective SERS nanosubstrates, and explore methods for the SERS detection of inorganic, organic, and biological contaminants including heavy metals, pharmaceuticals, plastic particles, synthetic dyes, pesticides, viruses, bacteria and mycotoxins. We also discuss the current limitations of SERS technologies in environmental detection and propose several avenues for future investigation. We encourage researchers to fill in the identified gaps so that SERS can be implemented in a real-world environment more effectively and efficiently, ultimately providing reliable and timely data to help and make science-based strategies and policies to protect environmental safety and public health.