Plasmonic nanoparticle sensors: current progress, challenges, and future prospects

Plasmonic nanoparticles (NPs) have played a significant role in the evolution of modern nanoscience and nanotechnology in terms of colloidal synthesis, general understanding of nanocrystal growth mechanisms, and their impact in a wide range of applications. They exhibit strong visible colors due to localized surface plasmon resonance (LSPR) that depends on their size, shape, composition, and the surrounding dielectric environment. Under resonant excitation, the LSPR of plasmonic NPs leads to a strong field enhancement near their surfaces and thus enhances various light-matter interactions. These unique optical properties of plasmonic NPs have been used to design chemical and biological sensors. Over the last few decades, colloidal plasmonic NPs have been greatly exploited in sensing applications through LSPR shifts (colorimetry), surface-enhanced Raman scattering, surface-enhanced fluorescence, and chiroptical activity. Although colloidal plasmonic NPs have emerged at the forefront of nanobiosensors, there are still several important challenges to be addressed for the realization of plasmonic NP-based sensor kits for routine use in daily life. In this comprehensive review, researchers of different disciplines (colloidal and analytical chemistry, biology, physics, and medicine) have joined together to summarize the past, present, and future of plasmonic NP-based sensors in terms of different sensing platforms, understanding of the sensing mechanisms, different chemical and biological analytes, and the expected future technologies. This review is expected to guide the researchers currently working in this field and inspire future generations of scientists to join this compelling research field and its branches.

Tetrahedral DNA-linked aptamer-antibody-based sandwich-type electrochemical sensor with Ag@Au core-shell nanoparticles as a signal amplifier for highly sensitive detection of α-fetoprotein

The conventional electrochemical detection strategy for alpha-fetoprotein (AFP) is limited by the antigen-antibody (Ag-Ab) reactions and suffers from low sensitivity and poor reproducibility due to the inconsistency of Ab-modified electrodes. Herein, we designed and explored a sandwich-type electrochemical sensor for highly sensitive detection of AFP based on aptamer (Apt)-AFP-Ab interaction mode with silver@gold (Ag@Au) core-shell nanoparticles (NPs) as a signal amplifier. AuNPs were electrodeposited onto MXene (Ti3C2TX)-modified glassy carbon electrode (GCE) to get AuNPs/MXene/GCE and further used as the signal amplification substrate. The tetrahedral DNA-linked AFP aptamers were immobilized onto AuNPs/MXene/GCE surface via Au-S bonds and used as the sensing and recognition platform for AFP capturing. Ag@AuNPs with core-shell structures were synthesized, characterized, and bound with Ab as detection elements by catalyzing H2O2 reduction. In the presence of AFP, a stable Apt-AFP-Ab sandwich structure was formed owing to the high affinities of aptamer and Ab toward the target AFP. The catalytic current produced by H2O2 reduction increased linearly with the logarithm of AFP concentration from 5 × 10-4 ng/mL to 1 × 105 ng/mL, accompanied by a low detection limit (1.6 × 10-4 ng/mL). Moreover, the novel sandwich-type electrochemical sensor shows high sensitivity, outstanding selectivity, and promising performance in the analysis of actual samples, displaying a broad application prospect in bioanalysis.

Tunable Preparation of SERS-Active Au-Ag Janus@Au NPs for Label-Free Staphylococcal Enterotoxin C Detection

Trace staphylococcal enterotoxin C (SEC) in food poses a serious risk to human health, and it is vital to develop a sensitive and accurate approach for SEC monitoring. Herein, a surface-enhanced Raman scattering (SERS) aptasensor was developed for the quantitative detection of SEC. SERS-active gold-silver Janus@gold nanoparticles (Au-Ag Janus@Au NPs) were prepared and showed tunable solid and hollow nanostructures by simply controlling the pH values of the reaction system. Solid Au-Ag Janus@Au NPs exhibited intrinsic and enhanced SERS activity due to the intense plasmonic coupling effect between Au dots and Au-Ag Janus NPs, which was 2.27-fold and 17.46-fold higher than that of Au-Ag Janus NPs and hollow Au-Ag Janus@Au NPs, respectively. The attachment of multiple Au dots also protected Ag islands from oxidization, which increased the stability of Au-Ag Janus@Au NPs. Solid Au-Ag Janus@Au NPs served as a label-free, strong, and stable SERS detection probe and achieved sensitive and reliable detection of SEC. The limit of detection was as low as 0.55 pg/mL. This study will expand the application prospects of label-free SERS detection probes in complex systems for food safety monitoring.

In Situ SERS Monitoring of the Plasmon-Driven Catalytic Reaction by Using Single Ag@Au Nanowires as Substrates

Single nanowires (NWs), as a kind of new surface-enhanced Raman scattering (SERS) substrates, have received extensive concern owing to their distinctive properties and distinct advantages. In this contribution, single Ag nanowires (AgNWs) and single Au-coated AgNWs (Ag@AuNWs) were fabricated by the laser-assisted pulling method and the galvanic replacement reaction, respectively. The prepared single Ag@AuNWs show both high SERS activity and catalytic activity through in situ monitoring and assessing the plasmon-driven surface-catalytic reaction of 4-nitrothiophenol (4-NTP) dimerizing to 4,4'-dimercaptoazobenzene and the reduction reaction of 4-NTP to para-aminothiophenol, respectively. It was found that the intensity of the Raman peak was affected greatly by the laser power, and the Raman peak could still be observed at 0.05% power under mild conditions (633 nm wavelength) in this single nanowire system. From the Raman spectrum, the SERS enhancement factor (EF) of 5.4 × 10⁴ can be obtained, and the EF value of 1.3 × 10⁹ can be reached at optimal conditions. Results have shown that single Ag@AuNWs can be utilized as a good platform for SERS applications with high sensitivity, stability, and reproducibility, which will benefit SERS-based research at the single entity level.

Multiplexed SERS Detection of Microcystins with Aptamer-Driven Core-Satellite Assemblies

We describe surface-enhanced Raman spectroscopy (SERS) aptasensors that can indirectly detect MC-LR and MC-RR, individually or simultaneously, in natural water and in algal culture. The sensor is constructed from nanoparticles composed of successive layers of Au core-SERS label-silver shell-gold shell (Au@label@Ag@Au NPs), functionalized on the outer Au surface by MC-LR and/or MC-RR aptamers. These NPs are immobilized on asymmetric Au nanoflowers (AuNFs) dispersed on planar silicon substrates through DNA hybridization of the aptamers and capture DNA sequences with which the AuNFs are functionalized, thereby forming core-satellite nanostructures on the substrates. This construction led to greater electromagnetic (EM) field enhancement of the Raman label-modified region, as supported by finite-difference time-domain (FDTD) simulations of the core-satellite assembly. In the presence of MC-LR and/or MC-RR, the aptamer-functionalized NPs dissociate from the AuNFs because of the stronger affinity of the aptamers with the MCs, which decreases the SERS signal, thus allowing indirect detection of the MCs. The improved SERS sensitivity significantly decreased the limit of detection (LOD) for separate MC-LR detection (0.8 pM) and for multiplex detection (1.5 pM for MC-LR and 1.3 pM for MC-RR), compared with other recently reported SERS-based methods for MC-LR detection. The aptasensors show excellent selectivity to MC-LR/MC-RR and excellent recoveries (96-105%). The use of these SERS aptasensors to monitor MC-LR production over 1 week in a culture medium of M. aeruginosa cells demonstrates the applicability of the sensors in a realistic environment.

DNA-Driven Nanoparticle Assemblies for Biosensing and Bioimaging

DNA molecules with superior flexibility, affinity and programmability have garnered considerable attention for the controllable assembly of nanoparticles (NPs). By controlling the density, length and sequences of DNA on NPs, the configuration of NP assemblies can be rationally designed. The specific recognition of DNA enables changes to be made to the spatial structures of NP assemblies, resulting in differences in tailorable optical signals. Comprehensive information on the fabrication of DNA-driven NP assemblies would be beneficial for their application in biosensing and bioimaging. This review analyzes the progress of DNA-driven NP assemblies, and discusses the tunable configurations determined by the structural parameters of DNA skeletons. The collective optical properties, such as chirality, fluorescence and surface enhanced Raman resonance (SERS), etc., of DNA-driven NP assemblies are explored, and engineered tailorable optical properties of these spatial structures are achieved. We discuss the development of DNA-directed NP assemblies for the quantification of DNA, toxins, and heavy metal ions, and demonstrate their potential application in the biosensing and bioimaging of tumor markers, RNA, living metal ions and phototherapeutics. We hihghlight possible challenges in the development of DNA-driven NP assemblies, and further direct potential prospects in the practical applications of macroscopical materials and photonic devices.

Present and Future of Surface-Enhanced Raman Scattering

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

Investigating the autophagy pathway in silver@gold core-shell nanoparticles-treated cells using surface-enhanced Raman scattering

Previous studies have shown that nanoparticles can induce autophagy, and the main approach for investigating autophagy induced by nanoparticles is via traditional methods such as TEM and biochemical assay. These methods measurements suffer from the disadvantages of complicated experimental processes, cell destruction, as well as lack of characterization of individual stages of the autophagy pathway. Surface-enhanced Raman scattering (SERS) has been extensively used in biological applications. With the combination of SERS and chemometric methods, such as principal component analysis-linear discriminant analysis (PCA-LDA), identification and distribution mapping of endosomes and lysosomes in the endocytosis of Au nanoparticles has been achieved by segregating the spectra from complex SERS data sets in the previous study. In this study, silver@gold core-shell nanoparticles (Ag@Au NPs) were synthesized by reduction of gold ions on the surface of the silver nanoparticles, and the autophagy induced by Ag@Au NPs was studied with Ag@Au NPs serving both as an autophagy inducer and as a high-performance SERS substrate. Pro-survival autophagy induced by Ag@Au NPs was proved by the western blot assay, flow cytometry and fluorescent staining. Furthermore, the autophagy pathway in Ag@Au NPs-treated cells was first elucidated by SERS combined with a modified reference-based PCA-LDA methodology. This study provides a feasible way of using SERS to elucidate the autophagy pathway induced by nanoparticles.

Fast detection of fenthion on fruit and vegetable peel using dynamic surface-enhanced Raman spectroscopy and random forests with variable selection

Dynamic surface-enhanced Raman spectroscopy (D-SERS) based on the state change of the substrate not only significantly enhances but also provides a highly reproducible Raman signal. Hence, we develop a fast and accurate method for the detection of fenthion on fruit and vegetable peel using D-SERS and random forests (RF) with variable selection. With uniform Ag nanoparticles, the dynamic spectra of fenthion solution at different concentrations were obtained using D-SERS, and fenthion solution greater than or equal to 0.05mg/L can be detected. Then, the quantitative analysis models of fenthion were developed by RF with variable selection for spectra of different range. The model of best performance is developed by RF and spectra of characteristic range with higher RF importance (top 40%), and the root mean square error of cross-validation is 0.0101mg/L. Moreover, the fenthion residue of tomato, pear, and cabbage peel were extracted by a swab dipped in ethanol and analyzed using the above method to further validate the practical effect. Compared to gas chromatography, the maximal relative deviation is below 12.5%, and the predicted recovery is between 87.5% and 112.5%. Accordingly, D-SERS and RF with variable selection can realize the fast, simple, ultrasensitive, and accurate analysis of fenthion residue on fruit and vegetable peel.

Nanotechnology-Enhanced No-Wash Biosensors for in Vitro Diagnostics of Cancer

In vitro biosensors have been an integral component for early diagnosis of cancer in the clinic. Among them, no-wash biosensors, which only depend on the simple mixing of the signal generating probes and the sample solution without additional washing and separation steps, have been found to be particularly attractive. The outstanding advantages of facile, convenient, and rapid response of no-wash biosensors are especially suitable for point-of-care testing (POCT). One fast-growing field of no-wash biosensor design involves the usage of nanomaterials as signal amplification carriers or direct signal generating elements. The analytical capacity of no-wash biosensors with respect to sensitivity or limit of detection, specificity, stability, and multiplexing detection capacity is largely improved because of their large surface area, excellent optical, electrical, catalytic, and magnetic properties. This review provides a comprehensive overview of various nanomaterial-enhanced no-wash biosensing technologies and focuses on the analysis of the underlying mechanism of these technologies applied for the early detection of cancer biomarkers ranging from small molecules to proteins, and even whole cancerous cells. Representative examples are selected to demonstrate the proof-of-concept with promising applications for in vitro diagnostics of cancer. Finally, a brief discussion of common unresolved issues and a perspective outlook on the field are provided.

Highly sensitive and selective colorimetric detection of sulphide using Ag–Au nanoalloys: a DFT study

For the colorimetric detection of S, SH, cysteine, and H₂S, the sensitivity of Ag–Au nanoalloy is better than pure Ag cluster, and both of them possess high selectivity for the detection of S.

New post-processing method of preparing nanofibrous SERS substrates with a high density of silver nanoparticles

The protocol to control density of AgNP on surfaces of nanofibers, and thus electromagnetic hotspots by variation of Tollens' reagent is established. Nanofiber films enable SERS either of solutes or macromolecular structures such as bacterial cells.

Double Detection of Mycotoxins Based on SERS Labels Embedded Ag@Au Core-Shell Nanoparticles

A sensitive surface-enhanced Raman scattering (SERS) signal dependent double detection of mycotoxins is achieved for the first time, without the aid of nucleic acid amplification strategies. SERS labels embedded Ag@Au core-shell (CS) nanoparticles (NPs) as novel SERS tags are successfully prepared through a galvanic replacement-free deposition. SERS tags produce stable and quantitative SERS signal, emerging from the plasmonic coupling at the junction of Ag core and Au shell. SERS tags engineered Raman aptasensors are developed for the double detection of ochratoxin A (OTA) and aflatoxin B1 (AFB1) in maize meal. The limits of detection (LODs) are as low as 0.006 ng/mL for OTA and 0.03 ng/mL for AFB1. The developed protocol can be extended to a large set of different SERS tags for the sensitive detection of multiple targets that possess different lengths of aptamers.

Au nanoflower–Ag nanoparticle assembled SERS-active substrates for sensitive MC-LR detection

Au nanoflower–Ag nanoparticle core–satellite assembly engineered Raman aptasensors achieved the sensitive detection of microcystin-LR in Tai lake water.

Dendritic Pt@Au nanowires as nanocarriers and signal enhancers for sensitive electrochemical detection of carcinoembryonic antigen

An electrochemical aptasensor for the sensitive and selective determination of carcinoembryonic antigen was constructed based on dendritic Pt@AuNWs as nanocarriers and signal enhancers.