Plasmon-Triggered Hot-Spot Excitation on SERS Substrates for Bacterial Inactivation and in Situ Monitoring

Recent advances in surface enhanced Raman spectroscopy for bacterial pathogen identifications

BACKGROUND

The rapid and reliable detection of pathogenic bacteria at an early stage is a highly significant research field for public health. However, most traditional approaches for pathogen identification are time-consuming and labour-intensive, which may cause physicians making inappropriate treatment decisions based on an incomplete diagnosis of patients with unknown infections, leading to increased morbidity and mortality. Therefore, novel methods are constantly required to face the emerging challenges of bacterial detection and identification. In particular, Raman spectroscopy (RS) is becoming an attractive method for rapid and accurate detection of bacterial pathogens in recent years, among which the newly developed surface-enhanced Raman spectroscopy (SERS) shows the most promising potential.

AIM OF REVIEW

Recent advances in pathogen detection and diagnosis of bacterial infections were discussed with focuses on the development of the SERS approaches and its applications in complex clinical settings.

KEY SCIENTIFIC CONCEPTS OF REVIEW

The current review describes bacterial classification using surface enhanced Raman spectroscopy (SERS) for developing a rapid and more accurate method for the identification of bacterial pathogens in clinical diagnosis. The initial part of this review gives a brief overview of the mechanism of SERS technology and development of the SERS approach to detect bacterial pathogens in complex samples. The development of the label-based and label-free SERS strategies and several novel SERS-compatible technologies in clinical applications, as well as the analytical procedures and examples of chemometric methods for SERS, are introduced. The computational challenges of pre-processing spectra and the highlights of the limitations and perspectives of the SERS technique are also discussed.Taken together, this systematic review provides an overall summary of the SERS technique and its application potential for direct bacterial diagnosis in clinical samples such as blood, urine and sputum, etc.

Quantitative detection of 6-thioguanine in body fluids based on a free-standing liquid membrane SERS substrate

The adverse reactions caused by 6-thioguanine (6-TG) in anti-cancer treatment are closely related to the dose, leading to the urgent need for clinical monitoring of its concentration. In this work, a highly reproducible free-standing liquid membrane (FLM) surface-enhanced Raman spectroscopy (SERS) substrate was developed to detect 6-TG in human urine and serum quantitatively. Briefly, a prepared sample was adjusted to pH 2 and mixed with concentrated core-shell bimetallic nanoparticle (Ag_coreAu_shell NP) suspension. The Au/Ag ratio of the Ag_coreAu_shell NPs was optimized. Then the mixture was formed into an FLM using a custom mold. The relative standard deviation (RSD) of the experimental results can be stabilized below 10% (n ≥ 10). The R² of the calibration curve in the range of 10 ~ 100 μg kg^-1 was 0.988. In addition, the limit of detection (LOD) (3σ/k) of 6-TG was 5 μg kg^-1. The FLM SERS platform has been successfully applied to the rapid and reliable analysis of 6-TG spiked in body fluids.

Nanomedicine as a promising strategy for the theranostics of infectious diseases

Infectious diseases caused by bacteria, viruses, and fungi and their global spread pose a great threat to human health. The 2019 World Health Organization report predicted that infection-related mortality will be similar to cancer mortality by 2050. Particularly, the global cumulative numbers of the recent outbreak of coronavirus disease (COVID-19) have reached 110.7 million cases and over 2.4 million deaths as of February 23, 2021. Moreover, the crisis of these infectious diseases exposes the many problems of traditional diagnosis, treatment, and prevention, such as time-consuming and unselective detection methods, the emergence of drug-resistant bacteria, serious side effects, and poor drug delivery. There is an urgent need for rapid and sensitive diagnosis as well as high efficacy and low toxicity treatments. The emergence of nanomedicine has provided a promising strategy to greatly enhance detection methods and drug treatment efficacy. Owing to their unique optical, magnetic, and electrical properties, nanoparticles (NPs) have great potential for the fast and selective detection of bacteria, viruses, and fungi. NPs exhibit remarkable antibacterial activity by releasing reactive oxygen species and metal ions, exerting photothermal effects, and causing destruction of the cell membrane. Nano-based delivery systems can further improve drug permeability, reduce the side effects of drugs, and prolong systemic circulation time and drug half-life. Moreover, effective drugs against COVID-19 are still lacking. Recently, nanomedicine has shown great potential to accelerate the development of safe and novel anti-COVID-19 drugs. This article reviews the fundamental mechanisms and the latest developments in the treatment and diagnosis of bacteria, viruses, and fungi and discusses the challenges and perspectives in the application of nanomedicine.

Integrated SERS Platform for Reliable Detection and Photothermal Elimination of Bacteria in Whole Blood Samples

Herein, an interference-free surface-enhanced Raman scattering (SERS) platform with a "sandwich" structure has been developed for reliable detection and photothermal killing of bacteria with whole blood as the real sample. The multifunctional platform comprised a plasmonic gold film (pAu) functionalized with bacteria-capturing units of 4-mercaptophenylboronic acid and internal reference of 4-mercaptobenzonitrile as the SERS substrate and vancomycin-modified core (gold)-shell (Prussian blue) nanoparticles (Au@PB@Van NPs) as the SERS tag. The detected SERS signals were from the Raman-silent region where no background signals occurred from biological sources, eliminating the interference and improving the detection sensitivity and accuracy. As a proof-of-concept, model bacterial strain, Staphylococcus aureus, was captured and detected in the whole blood samples. Furthermore, high antibacterial efficiency of approximately 100% was reached under the synergistic photothermal effect from pAu and Au@PB@Van NPs. This study provides a new avenue for bacteria detection in real samples and their subsequent in situ elimination.

Bacteria Detection: From Powerful SERS to Its Advanced Compatible Techniques

The rapid, highly sensitive, and accurate detection of bacteria is the focus of various fields, especially food safety and public health. Surface-enhanced Raman spectroscopy (SERS), with the advantages of being fast, sensitive, and nondestructive, can be used to directly obtain molecular fingerprint information, as well as for the on-line qualitative analysis of multicomponent samples. It has therefore become an effective technique for bacterial detection. Within this progress report, advances in the detection of bacteria using SERS and other compatible techniques are discussed in order to summarize its development in recent years. First, the enhancement principle and mechanism of SERS technology are briefly overviewed. The second part is devoted to a label-free strategy for the detection of bacterial cells and bacterial metabolites. In this section, important considerations that must be made to improve bacterial SERS signals are discussed. Then, the label-based SERS strategy involves the design strategy of SERS tags, the immunomagnetic separation of SERS tags, and the capture of bacteria from solution and dye-labeled SERS primers. In the third part, several novel SERS compatible technologies and applications in clinical and food safety are introduced. In the final part, the results achieved are summarized and future perspectives are proposed.

A novel nucleic acid amplification system based on nano-gap embedded active disk resonators

Recent advances in nucleic acid based testing using bio-optical sensor approaches have been introduced but most are based on hybridization between the optical sensor and the bio-molecule and not on an amplification mechanism. Direct nucleic acid amplification on an optical sensor has several technical limitations, such as the sensitivity of the temperature sensor, instrument complexity, and high background signal. We here describe a novel nucleic acid amplification method based on a whispering gallery mode active resonator and discuss its potential molecular diagnostic application. By implanting nanoclusters as active compounds, this active resonator operates without tapered fiber coupling and emits a strong photoluminescence signal with low background in the wavelength of low absorption in an aqueous environment that is typical of biosensors. Our method also offers an extremely low detection threshold down to a single copy within 10 min due to the strong light-matter interaction in a nano-gap structure. We envision that this active resonator provides a high refractive index contrast for tight mode confinement with simple alignment as well as the possibility of reducing the device size so that a point-of-care system with low-cost, high-sensitivity and simplicity.

Boosting the Raman signal on a semiconductor-nanotube membrane for reporting photocatalytic reactions on site

A smooth TiO₂ nanotube-based membrane (S-TiNM) is demonstrated to be effective in reducing the Raman-scattering loss in 3D porous structures. Using the photocatalytic reduction of 4-nitrothiophenol as a model, the S-TiNMs show promising potential as sensitive SERS substrates to investigate photocatalytic reactions on site.

Highly Sensitive Surface-Enhanced Raman Spectroscopy Substrates of Ag@PAN Electrospinning Nanofibrous Membranes for Direct Detection of Bacteria

Surface-enhanced Raman spectroscopy (SERS) can be applied for biological detection because of its high sensitivity and noninvasiveness for analytes. Herein, we engineer plasmonic free-standing substrates composed of Ag nanoparticles (NPs) supported on polyacrylonitrile (PAN) electrospinning nanofibrous felts as sensors for bacterial detection. Ag NPs are evenly distributed on PAN nanofibers after preimpregnation and impregnation of PAN nanofibers in Tollens' reagent. The size and loading density of Ag NPs are tunable by adjusting the reaction time of glucose and Tollens' reagent, thereby allowing the tuning of the surface plasmon resonance. Using 4-mercaptophenol (4-MPh) and 4-mercaptobenzoic acid (4-MBA) as probe molecules, SERS effects of Ag@PAN composite nanofibers are investigated, and the substrates allow the detection of 4-MPh and 4-MBA at a low concentration of 10-9 mol/L. Importantly, the substrates exhibit a high sensitivity of SERS performance for bacterial identification without a specific bacteria-aptamer conjugation. The SERS substrates also show good uniformity of SERS response for bacterial organelles. Furthermore, the antimicrobial property was evaluated, and the results indicate that the sample of Ag@PAN nanofiber mats possesses excellent antibacterial properties against Escherichia coli and Staphylococcus aureus.

Synthesis of Ag@CuO nanohybrids and their photo-enhanced bactericidal effect through concerted Ag ion release and reactive oxygen species generation

Ag and CuO in the form of nanoparticles have been widely used in our daily life as antibacterial agents, through releasing Ag ions and generating reactive oxygen species (ROS). In this work, we demonstrate that by synthesizing Ag@CuO nanohybrids with core-shell configurations, their bactericidal activity can be synergistically enhanced compared to the respective constituents. Upon AM 1.5G light illumination for short durations, the measured minimum inhibitory concentrations of the Ag@CuO nanohybrids show a significant decrease against both Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus bacterial strains, requiring only 7% and 34% of those when conducted in the dark. The spread plate results demonstrate that with nanohybrid concentrations of 11.0 and 21.9 mg L^-1, at least 7 orders of magnitude decrease in Escherichia coli and Staphylococcus aureus colony forming units is achieved, when the Ag@CuO nanohybrids are exposed to light illumination for 10 min. The effect of illumination is found to induce Ag⁺ release and enhance ¹O₂ generation, which act concertedly to facilitate the remarkable photo-enhanced bactericidal effect.

Upconversion Nanoparticle-Assisted Payload Delivery from TiO2 under Near-Infrared Light Irradiation for Bacterial Inactivation

The low penetration depth of UV light in mammalian tissue is the critical limitation for the use of TiO₂-based photocatalysis in biomedical applications. In this work, we develop an effective near-infrared (NIR)-active photocatalytic platform that consists of a shell structure of upconversion nanocrystals decorated on a core of Au/dark-TiO₂. The heart of this system is the strong photocatalytic activity in the visible region enabled by the gold surface-plasmon resonance on dark TiO₂ (D-TiO₂). Simulation and experiment demonstrate for an optimized Au/D-TiO₂ combination a highly enhanced light absorption in the visible range. Using ampicillin sodium (AMP) as model drug, we exemplify the effective use of this principle by demonstrating a NIR light-triggered photocatalytic payload release. Importantly, the photocatalytically generated reactive oxygen species can effectively inactivate AMP-resistant bacteria strains, thus maintaining an antibacterial effect even after all drug is released. Overall, we anticipate that the here-introduced NIR-light-active photocatalytic cascade can considerably widen TiO₂-based photocatalysis and its applications into the infrared range.

Electrical Tuning of the SERS Enhancement by Precise Defect Density Control

Surface-enhanced Raman scattering (SERS) has been widely established as a powerful analytical technique in molecular fingerprint recognition. Although conventional noble metal-based SERS substrates show admirable enhancement of the Raman signals, challenges on reproducibility, biocompatibility, and costs limit their implementations as the preferred analysis platforms. Recently, researches on SERS substrates have found that some innovatively prepared metal oxides/chalcogenides could produce noble metal comparable SERS enhancement, which profoundly expanded the material selection. Nevertheless, to tune the SERS enhancement of these materials, careful experimental designs and sophisticated processes were needed. Here, an electrically tunable SERS substrate based on tungsten oxides (WO_3-x) is demonstrated. An electric field is used to introduce the defects in the oxide on an individual substrate, readily invoking the SERS detection capability, and further tuning the enhancement factor is achieved through electrical programming of the oxide leakage level. Additionally, by virtue of in situ tuning the defect density and enhancement factor, the substrate can adapt to different molecular concentrations, potentially improving the detection range. These results not only help build a better understanding of the chemical mechanism but also open an avenue for engaging non-noble metal materials as multifunctional SERS substrates.