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.

Advancing COVID-19 Diagnosis: Enhancement in SERS-PCR with 30-nm Au Nanoparticle-Internalized Nanodimpled Substrates.

Real-time polymerase chain reaction (RT-PCR) with fluorescence detection is the gold standard for diagnosing coronavirus disease 2019 (COVID-19) However, the fluorescence detection in RT-PCR requires multiple amplification steps when the initial deoxyribonucleic acid (DNA) concentration is low. Therefore, this study has developed a highly sensitive surface-enhanced Raman scattering-based PCR (SERS-PCR) assay platform using the gold nanoparticle (AuNP)-internalized gold nanodimpled substrate (AuNDS) plasmonic platform. By comparing different sizes of AuNPs, it is observed that using 30 nm AuNPs improves the detection limit by approximately ten times compared to 70 nm AuNPs. Finite-difference time-domain (FDTD) simulations show that multiple hotspots are formed between AuNPs and the cavity surface and between AuNPs when 30 nm AuNPs are internalized in the cavity, generating a strong electric field. With this 30 nm AuNPs-AuNDS SERS platform, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) ribonucleic acid (RNA)-dependent RNA polymerase (RdRp) can be detected in only six amplification cycles, significantly improving over the 25 cycles required for RT-PCR. These findings pave the way for an amplification-free molecular diagnostic system based on SERS.

SERS-based microdevices for use as in vitro diagnostic biosensors.

Advances in surface-enhanced Raman scattering (SERS) detection have helped to overcome the limitations of traditional in vitro diagnostic methods, such as fluorescence and chemiluminescence, owing to its high sensitivity and multiplex detection capability. However, for the implementation of SERS detection technology in disease diagnosis, a SERS-based assay platform capable of analyzing clinical samples is essential. Moreover, infectious diseases like COVID-19 require the development of point-of-care (POC) diagnostic technologies that can rapidly and accurately determine infection status. As an effective assay platform, SERS-based bioassays utilize SERS nanotags labeled with protein or DNA receptors on Au or Ag nanoparticles, serving as highly sensitive optical probes. Additionally, a microdevice is necessary as an interface between the target biomolecules and SERS nanotags. This review aims to introduce various microdevices developed for SERS detection, available for POC diagnostics, including LFA strips, microfluidic chips, and microarray chips. Furthermore, the article presents research findings reported in the last 20 years for the SERS-based bioassay of various diseases, such as cancer, cardiovascular diseases, and infectious diseases. Finally, the prospects of SERS bioassays are discussed concerning the integration of SERS-based microdevices and portable Raman readers into POC systems, along with the utilization of artificial intelligence technology.

Highly Uniform Self-Assembly of Gold Nanoparticles by Butanol-Induced Dehydration and Its SERS Applications in SARS-CoV-2 Detection.

We report the development of a reproducible and highly sensitive surface-enhanced Raman scattering (SERS) substrate using a butanol-induced self-assembly of gold nanoparticles (AuNPs) and its application as a rapid diagnostic platform for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The butanol-induced self-assembly process was used to generate a uniform assembly of AuNPs, with multiple hotspots, to achieve high reproducibility. When an aqueous droplet containing AuNPs and target DNAs was dropped onto a butanol droplet, butanol-induced dehydration occurred, enriching the target DNAs around the AuNPs and increasing the loading density of the DNAs on the AuNP surface. The SERS substrate was evaluated by using Raman spectroscopy, which showed strong electromagnetic enhancement of the Raman signals. The substrate was then tested for the detection of SARS-CoV-2 using SERS, and a very low limit of detection (LoD) of 3.1 × 10-15 M was obtained. This provides sufficient sensitivity for the SARS-CoV-2 screening assay, and the diagnostic time is significantly reduced as no thermocycling steps are required. This study demonstrates a method for the butanol-induced self-assembly of AuNPs and its application as a highly sensitive and reproducible SERS substrate for the rapid detection of SARS-CoV-2. The results suggest the potential of this approach for developing rapid diagnostic platforms for other biomolecules and infectious diseases.

Rapid and Accurate On-Site Immunodiagnostics of Highly Contagious Severe Acute Respiratory Syndrome Coronavirus 2 Using Portable Surface-Enhanced Raman Scattering-Lateral Flow Assay Reader.

In early 2022, the number of people infected with the highly contagious mutant severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), called Omicron, was increasing worldwide. Therefore, several countries approved the lateral flow assay (LFA) strip as a diagnostic method for confirming SARS-CoV-2 instead of reverse transcription-polymerase chain reaction (RT-PCR), which takes a long time to generate the results. However, owing to the limitation of detection sensitivity, commercial LFA strips have high false-negative diagnosis rates for patients with low virus concentrations. Therefore, in this study, we developed a portable surface-enhanced Raman scattering (SERS)-LFA reader based on localized surface plasmon effects to solve the sensitivity problem of the commercial LFA strip. We tested 54 clinical samples using this portable SERS-LFA reader, which generated 49 positive and 5 negative results. Out of the 49 positive results, SERS-LFA classified only 2 as false negative, while the commercial LFA classified 21 as false negative. This confirmed that the false-negative rate had significantly improved compared to that of commercial LFA strips. We believe that the proposed SERS-LFA system can be utilized as a point-of-care diagnostic system to quickly and accurately determine a virus infection that could spread significantly within a short period.

Dual-mode SERS-based lateral flow assay strips for simultaneous diagnosis of SARS-CoV-2 and influenza a virus.

Since COVID-19 and flu have similar symptoms, they are difficult to distinguish without an accurate diagnosis. Therefore, it is critical to quickly and accurately determine which virus was infected and take appropriate treatments when a person has an infection. This study developed a dual-mode surface-enhanced Raman scattering (SERS)-based LFA strip that can diagnose SARS-CoV-2 and influenza A virus with high accuracy to reduce the false-negative problem of the commercial colorimetric LFA strip. Furthermore, using a single strip, it is feasible to detect SARS-CoV-2 and influenza A virus simultaneously. A clinical test was performed on 39 patient samples (28 SARS-CoV-2 positives, 6 influenza A virus positives, and 5 negatives), evaluating the clinical efficacy of the proposed dual-mode SERS-LFA strip. Our assay results for clinical samples show that the dual-mode LFA strip significantly reduced the false-negative rate for both SARS-CoV-2 and influenza A virus.

SERS-based dual-mode DNA aptasensors for rapid classification of SARS-CoV-2 and influenza A/H1N1 infection.

We developed a dual-mode surface-enhanced Raman scattering (SERS)-based aptasensor that can accurately diagnose and distinguish severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and influenza A/H1N1 at the same time. Herein, DNA aptamers that selectively bind to SARS-CoV-2 and influenza A/H1N1 were immobilized together on Au nanopopcorn substrate. Raman reporters (Cy3 and RRX), attached to the terminal of DNA aptamers, could generate strong SERS signals in the nanogap of the Au nanopopcorn substrate. Additionally, the internal standard Raman reporter (4-MBA) was immobilized on the Au nanopopcorn substrate along with aptamer DNAs to reduce errors caused by changes in the measurement environment. When SARS-CoV-2 or influenza A virus approaches the Au nanopopcorn substrate, the corresponding DNA aptamer selectively detaches from the substrate due to the significant binding affinity between the corresponding DNA aptamer and the virus. As a result, the related SERS intensity decreases with increasing target virus concentration. Thus, it is possible to determine whether a suspected patient is infected with SARS-CoV-2 or influenza A using this SERS-based DNA aptasensor. Furthermore, this sensor enables a quantitative evaluation of the target virus concentration with high sensitivity without being affected by cross-reactivity. Therefore, this SERS-based diagnostic platform is considered a conceptually new diagnostic tool that rapidly discriminates against these two respiratory diseases to prevent their spread.

Surface-enhanced Raman scattering-based immunoassay for severe acute respiratory syndrome coronavirus 2.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has affected humans worldwide for over a year now. Although various tests have been developed for the detection of SARS-CoV-2, advanced sensing methods are required for the diagnosis, screening, and surveillance of coronavirus disease 2019 (COVID-19). Here, we report a surface-enhanced Raman scattering (SERS)-based immunoassay involving an antibody pair, SERS-active hollow Au nanoparticles (NPs), and magnetic beads for the detection of SARS-CoV-2. The selected antibody pair against the SARS-CoV-2 antigen, along with the magnetic beads, facilitates the accurate direct detection of the virus. The hollow Au NPs exhibit strong, reproducible SERS signals, allowing sensitive quantitative detection of SARS-CoV-2. This assay had detection limits of 2.56 fg/mL for the SARS-CoV-2 antigen and 3.4 plaque-forming units/mL for the SARS-CoV-2 lysates. Furthermore, it facilitated the identification of SARS-CoV-2 in human nasopharyngeal aspirates and diagnosis of COVID-19 within 30 min using a portable Raman device. Thus, this assay can be potentially used for the diagnosis and prevention of COVID-19.

Surface-Enhanced Raman Scattering-Based Dual-Flow Lateral Flow Assay Sensor for the Ultrasensitive Detection of the Thyroid-Stimulating Hormone.

The surface-enhanced Raman scattering (SERS)-based lateral flow assay (LFA) strip has been considered a high-sensitivity sensor that can overcome the low sensitivity and the difficulty of quantitative analysis problems inherent in the colorimetric LFA sensor. In the SERS-based LFA strip reported so far, a liquid sample flows through the nitrocellulose membrane in a single pathway. In some cases, however, this single-flow approach still has a limitation in detection sensitivity. This study developed a conceptually new SERS-based dual-flow LFA sensor to improve the detection sensitivity in a single-flow LFA sensor. First, a 25 nm Raman reporter-labeled gold nanoparticle (AuNP) solution flowed through one way, and a 45 nm AuNP solution continuously flowed through another path. This sequential flow of two different AuNP solutions enables forming additional bright hot spots between 25 and 45 nm AuNPs in the test line, and the SERS signal is strongly enhanced. Using this SERS-based dual-flow LFA sensor, it was possible to detect thyroid-stimulating hormone less than 0.5 µIU/mL that cannot be measured with a SERS-based single-flow LFA sensor.

Recent advances in sensitive surface-enhanced Raman scattering-based lateral flow assay platforms for point-of-care diagnostics of infectious diseases.

This review reports the recent advances in surface-enhanced Raman scattering (SERS)-based lateral flow assay (LFA) platforms for the diagnosis of infectious diseases. As observed through the recent infection outbreaks of COVID-19 worldwide, a timely diagnosis of the disease is critical for preventing the spread of a disease and to ensure epidemic preparedness. In this regard, an innovative point-of-care diagnostic method is essential. Recently, SERS-based assay platforms have received increasing attention in medical communities owing to their high sensitivity and multiplex detection capability. In contrast, LFAs provide a user-friendly and easily accessible sensing platform. Thus, the combination of LFAs with a SERS detection system provides a new diagnostic modality for accurate and rapid diagnoses of infectious diseases. In this context, we briefly discuss the recent application of LFA platforms for the POC diagnosis of SARS-CoV-2. Thereafter, we focus on the recent advances in SERS-based LFA platforms for the early diagnosis of infectious diseases and their applicability for the rapid diagnosis of SARS-CoV-2. Finally, the key issues that need to be addressed to accelerate the clinical translation of SERS-based LFA platforms from the research laboratory to the bedside are discussed.