Multiplexed detection of serological cancer markers with plasmon-enhanced Raman spectro-immunoassay

Putting Hybrid Nanomaterials to Work for Biomedical Applications

Hybrid nanomaterials have found use in many biomedical applications. This article provides a comprehensive review of the principles, techniques, and recent advancements in the design and fabrication of hybrid nanomaterials for biomedicine. We begin with an introduction to the general concept of material hybridization, followed by a discussion of how this approach leads to materials with additional functionality and enhanced performance. We then highlight hybrid nanomaterials in the forms of nanostructures, nanocomposites, metal-organic frameworks, and biohybrids, including their fabrication methods. We also showcase the use of hybrid nanomaterials to advance biomedical engineering in the context of nanomedicine, regenerative medicine, diagnostics, theranostics, and biomanufacturing. Finally, we offer perspectives on challenges and opportunities.

Diatomite-Based, Flexible SERS Immunosensor Platform for Rapid, Specific, and Sensitive Detection of Circulating Cancer-Specific Protein Biomarkers in Serum Using Raman Probes

Cancer is one of the most actively researched diseases having a high mortality rate when not detected at an early stage. Thus, rapid, simultaneous, and sensitive quantification of cancer biomarkers plays an important role in early diagnosis, with patient impact to disability adjusted life years. Herein, a diatomite-based SERS flexible platform for the rapid and sensitive detection of circulating cancer-specific protein biomarkers in serum is presented. In this approach, diatomite/AgNPs strips with maximum SERS activity prepared using the layer-by-layer (LbL) technique were modified with specific antibodies, and specific antigens (HER2, CA15-3, PSA, and MUC4) were captured and detected. By using Raman probes specific to the captured antigens in serum, a SERS limit of detection (LOD) of 0.1 ng/mL was measured (calculated LOD < 0.1 ng/mL). This value is lower than the cutoff amount of cancer antigens in the person's blood. The specificity for the antigens of each antibody was calculated to be higher than 95%. As a result, an immunosensor for rapid detection of cancer biomarkers in serum with good specificity, high sensitivity, good reproducibility, and low cost has been demonstrated. Overall, we show that the prepared diatomite-based SERS substrate with a high surface-to-volume ratio is a useable platform for immunoassay tests.

Self-calibrating surface-enhanced Raman scattering-lateral flow immunoassay for determination of amyloid-β biomarker of Alzheimer's disease

Rapid early diagnosis of Alzheimer's disease (AD) is critical for its effective and prompt treatment since the clinically available treatments can only relieve the symptoms or slow the disease progression. However, it is still a grand challenge to accurately diagnose AD at its early stage because of the indiscernible early symptoms and the lack of sensitive detection tools. Here, we develop a self-calibrating surface-enhanced Raman scattering (SERS)-lateral flow immunoassay (LFIA) biosensor for quantitative analysis of amyloid-β1-42 (Aβ1-42) biomarker in biofluids, enabling accurate AD diagnosis. The designed SERS-LFIA biosensor makes full use of the unique aspects of the LFIA format and the SERS technique to quantify the Aβ1-42 level in complex biofluids with high sensitivity, excellent anti-interference capability, low-cost, and operation simplicity. The key aspect of the design of this biosensor is that internal standard (IS)-SERS nanoparticles are embedded in the test line of the test strip as a self-calibration unit for correction of fluctuations of SERS signals caused by various external factors such as test parameters and sample heterogeneity. We demonstrate significant improvement of the detection performance of the SERS-LFIA biosensor for ratiometric quantification of Aβ1-42 owing to the built-in IS in the test line. We expect that the present IS-based biosensing strategy provides a promising tool for accurate AD diagnosis and longitudinal monitoring of therapeutic response with great promises for clinical translation.

SERS immuno- and apta-assays in biosensing/bio-detection: Performance comparison, clinical applications, challenges

Surface Enhanced Raman Spectroscopy is increasingly used as a sensitive bioanalytical tool for detection of variety of analytes ranging from viruses and bacteria to cancer biomarkers and toxins, etc. This comprehensive review describes principles of operation and compares the performance of immunoassays and aptamer assays with Surface Enhanced Raman scattering (SERS) detection to each other and to some other bioassay methods, including ELISA and fluorescence assays. Both immuno- and aptamer-based assays are categorized into assay on solid substrates, assays with magnetic nanoparticles and assays in laminar flow or/and strip assays. The best performing and recent examples of assays in each category are described in the text and illustrated in the figures. The average performance, particularly, limit of detection (LOD) for each of those methods reflected in 9 tables of the manuscript and average LODs are calculated and compared. We found out that, on average, there is some advantage in terms of LOD for SERS immunoassays (0.5 pM median LOD of 88 papers) vs SERS aptamer-based assays (1.7 pM median LOD of 51 papers). We also tabulated and analyzed the clinical performance of SERS immune and aptamer assays, where selectivity, specificity, and accuracy are reported, we summarized the best examples. We also reviewed challenges to SERS bioassay performance and real-life application, including non-specific protein binding, nanoparticle aggregation, limited nanotag stability, sometimes, relatively long time to results, etc. The proposed solutions to those challenges are also discussed in the review. Overall, this review may be interesting not only to bioanalytical chemist, but to medical and life science researchers who are interested in improvement of bioanalyte detection and diagnostics.

SERS Immunosensors for Cancer Markers Detection

Early diagnosis and monitoring are essential for the effective treatment and survival of patients with different types of malignancy. To this end, the accurate and sensitive determination of substances in human biological fluids related to cancer diagnosis and/or prognosis, i.e., cancer biomarkers, is of ultimate importance. Advancements in the field of immunodetection and nanomaterials have enabled the application of new transduction approaches for the sensitive detection of single or multiple cancer biomarkers in biological fluids. Immunosensors based on surface-enhanced Raman spectroscopy (SERS) are examples where the special properties of nanostructured materials and immunoreagents are combined to develop analytical tools that hold promise for point-of-care applications. In this frame, the subject of this review article is to present the advancements made so far regarding the immunochemical determination of cancer biomarkers by SERS. Thus, after a short introduction about the principles of both immunoassays and SERS, an extended presentation of up-to-date works regarding both single and multi-analyte determination of cancer biomarkers is presented. Finally, future perspectives on the field of SERS immunosensors for cancer markers detection are briefly discussed.

A clinically feasible diagnostic spectro-histology built on SERS-nanotags for multiplex detection and grading of breast cancer biomarkers

Simultaneous detection of multiple biomarkers is always an obstacle in immunohistochemical (IHC) analysis. Herein, a straightforward spectroscopy-driven histopathologic approach has emerged as a paradigm of Raman-label (RL) nanoparticle probes for multiplex recognition of pertinent biomarkers in heterogeneous breast cancer. The nanoprobes are constructed by sequential incorporation of signature RL and target specific antibodies on gold nanoparticles, which are coined as Raman-Label surface enhanced Raman scattering (RL-SERS)-nanotags to evaluate simultaneous recognition of clinically relevant breast cancer biomarkers i.e., estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor2 (HER2). As a foot-step assessment, breast cancer cell lines having varied expression levels of the triple biomarkers are investigated. Subsequently, the optimized detection strategy using RL-SERS-nanotags is subjected to clinically confirmed, retrospective formalin-fixed paraffin embedded (FFPE) breast cancer tissue samples to fish out the quick response of singleplex, duplex as well as triplex biomarkers in a single tissue specimen by adopting a ratiometric signature RL-SERS analysis which enabled to minimize the false negative and positive results. Significantly, sensitivity and specificity of 95% and 92% for singleplex, 88% and 85% for duplex, and 75% and 67% for triplex biomarker has been achieved by assessing specific Raman fingerprints of the respective SERS-tags. Furthermore, a semi-quantitative evaluation of HER2 grading between 4+/2+/1+ tissue samples was also achieved by the Raman intensity profiling of the SERS-tag, which is fully in agreement with the expensive fluorescent in situ hybridization analysis. Additionally, the practical diagnostic applicability of RL-SERS-tags has been achieved by large area SERS imaging of areas covering 0.5-5 mm2 within 45 min. These findings unveil an accurate, inexpensive and multiplex diagnostic modality envisaging large-scale multi-centric clinical validation.

Disposable label-free electrochemical immunosensor based on prussian blue nanocubes for four breast cancer tumor markers

A compact and low-cost multi-electrode array (MEA) is presented, comprising four working electrodes with shared reference and auxiliary electrodes. Prussian blue was electrodeposited on the MEA using chronoamperometry with a positive potential of 0.3 V. Prussian blue nanocubes (PBNCs) were formed, which were observed using scanning electron microscopy. The precision of the four working electrodes was demonstrated using ferric/ferro cyanide (RSD <5.8%). The surface roughness of the working electrodes of the fabricated MEA was investigated by atomic force microscopy and compared with that of a commercial MEA. The PBNCs were the platform for a label-free immunosensor that detected four breast cancer tumor markers (CEA, CA125, CA153, and CA199) using specific antibodies. The processes of antibody immobilization were investigated using cyclic voltammetry and electrochemical impedance spectroscopy. The immunosensor was evaluated using real human serum samples, yielding acceptable recoveries (95.1-104.1%, RSD < 3.9) for the four tumor markers. These findings confirmed that our label-free immunosensor based on PBNCs could be a promising device for point-of-care testing and could pave the way for the establishment of new platforms for the screening of various breast cancer tumor markers.

Bimetallic alloy Ag@Au nanorings with hollow dual-rims focus near-field on circular intra-nanogaps

Here, we report a highly sensitive and reliable surface enhanced Raman scattering (SERS)-based immunoassay using bimetallic alloy Ag@Au hollow dual-rim nanorings (DRNs) where two hollow nanorings with different diameters are concentrically overlapped and connected by thin metal ligaments, forming circular hot-zones in the intra-nanogaps between the inner and outer rims. Pt DRNs were first prepared, and then Ag was deposited on the surface of the Pt skeleton, followed by Au coating, resulting in alloy Ag@Au hollow DRNs. The chemical stability of Au and the high optical properties of Ag are incorporated into a single entity, Ag@Au hollow DRNs, enabling strong single-particle SERS activity and biocompatibility through surface modification with thiol-containing functionalities. When Ag@Au hollow DRNs were utilized as nanoprobes for detecting human chorionic gonadotropin (HCG) hormone through a SERS-based immunoassay, a very low limit of detection of 10 pM with high reliability was achieved, strongly indicating their advantage as ultrasensitive SERS nanoprobes.

Multiple Biomarker Simultaneous Detection in Serum via a Nanomaterial-Functionalized Biosensor for Ovarian Tumor/Cancer Diagnosis

Ovarian tumors/cancers are threatening women's health worldwide, which demands high-performance detection methods and accurate strategies to effectively detect, diagnose and treat them. Here, we report a nanographene oxide particle-functionalized microfluidic fluorescence biosensor to simultaneously detect four biomarkers, CA125, HE4, CEA and APF, for ovarian tumor/cancer diagnosis. The developed biosensor exhibits good selectivity and a large biomarker detection range with a limit of detection of 0.01 U/mL for CA125 and ~1 pg/mL for HE4, CEA and APF. The current results indicate that (1) the proposed biosensor is a promising tool for the simultaneous detection of multiple biomarkers in ovarian tumors/cancer and (2) CA125 and HE4 are strong indicators, AFP may be helpful, and CEA is a weak biomarker for ovarian tumor/cancer diagnosis. The proposed biosensor would be a potential tool, and an analytical approach for the simultaneous detection of multiple biomarkers will provide a new strategy for the early screening, diagnosis and treatment of ovarian tumors/cancers, as well as other cancers.

Optical biosensor based on SERS with signal calibration function for quantitative detection of carcinoembryonic antigen

Monitoring the levels of cancer biomarkers is essential for cancer diagnosis and evaluation. In this study, a novel sandwich type sensing platform based on surface-enhanced Raman scattering (SERS) technology was developed for the detection of carcinoembryonic antigen (CEA), with a limit of detection (LOD) of 0.258 ng/mL. In order to achieve sensitive detection of CEA in complex samples, gold nanoparticle monolayer modified with CEA antibodies and with aptamer-functionalized probes was fabricated to target CEA. Two gold layers were integrated into the SERS platform, which greatly enhanced the signal of the probe by generating tremendous "hot spots". Meanwhile, the intensity ratio of Raman probes and the second-order peak of the silicon wafer was used to achieve dynamic calibration of the Raman probe signal. Excitingly, this sensing platform was capable of distinguishing cancer patients from healthy individuals via CEA concentrations in blood samples with the accuracy of 100%. This sandwich structure SERS sensing platform presented promising potential to be an alternative tool for clinical biomarker detection in the field of cancer diagnosis.

Leveraging Nanomechanical Perturbations in Raman Spectro-Immunoassays to Design a Versatile Serum Biomarker Detection Platform

While immunoassays are pivotal to medical diagnosis and bioanalytical chemistry, the current landscape of public health has catalyzed an important shift in the requirements of immunoassays that demand innovative solutions. For example, rapid, label-free, and low-cost screening of a given analyte is required to inform the best countermeasures to combat infectious diseases in a timely manner. Yet, the current design of immunoassays cannot accommodate such requirements as constraint by accumulative challenges, such as repeated incubation and washing, and the need of two types of antibodies in the sandwich format. To provide a potential solution, herein, a plasmonic Raman immunoassay with single-antibody, multivariate regression, and shift-of-peak strategies, coined as the PRISM assay, for serum biomarkers detection, is reported. The PRISM assay relies on Raman reporter-antibody conjugates to capture analytes on a plasmonic substrate. The ensuing nanomechanical perturbations to vibration of Raman reporters induce subtle but characteristic spectral changes that encode rich information related to the captured analytes. By fusing Raman spectroscopy and chemometric analysis, both Raman frequency shift- and multivariate regression models for sensitive detection of biomarkers are developed. The PRISM assay is expected to find a wide range of applications in clinical diagnosis, food safety surveillance, and environmental monitoring.

Gold nanocrystals: optical properties, fine-tuning of the shape, and biomedical applications

Noble metal nanomaterials with special physical and chemical properties have attracted considerable attention in the past decades. In particular, Au nanocrystals (NCs), which possess high chemical inertness and unique surface plasmon resonance (SPR), have attracted extensive research interest. In this study, we review the properties and preparation of Au NCs with different morphologies as well as their important applications in biological detection. The preparation of Au NCs with different shapes by many methods such as seed-mediated growth method, seedless synthesis, polyol process, ultrasonic method, and hydrothermal treatment has already been introduced. In the seed-mediated growth method, the influence factors in determining the final shape of Au NCs are discussed. Au NCs, which show significant size-dependent color differences are proposed for preparing biological probes to detect biomacromolecules such as DNA and protein, while probe conjugate molecules serves as unique coupling agents with a target. Particularly, Au nanorods (NRs) have some unique advantages in the application of biological probes and photothermal cancer therapy compared to Au nanoparticles (NPs).

Gold Nanoparticles Based Optical Biosensors for Cancer Biomarker Proteins: A Review of the Current Practices

Cancer prognosis depends on the early detection of the disease. Gold nanoparticles (AuNPs) have attracted much importance in biomedical research due to their distinctive optical properties. The AuNPs are easy to fabricate, biocompatible, surface controlled, stable, and have surface plasmonic properties. The AuNPs based optical biosensors can intensely improve the sensitivity, specificity, resolution, penetration depth, contrast, and speed of these devices. The key optical features of the AuNPs based biosensors include localized surface plasmon resonance (LSPR), SERS, and luminescence. AuNPs based biomarkers have the potential to sense the protein biomarkers at a low detection level. In this review, the fabrication techniques of the AuNPs have been reviewed. The optical biosensors based on LSPR, SERS, and luminescence are also evaluated. The application of these biosensors for cancer protein detection is discussed. Distinct examples of cancer research that have a substantial impact on both scientific and clinical research are presented.

Recent Advances in Sandwich SERS Immunosensors for Cancer Detection

Cancer has been one of the most prevalent diseases around the world for many years. Its biomarkers are biological molecules found in the blood or other body fluids of people with cancer diseases. These biomarkers play a crucial role not only in the diagnosis of cancer diseases, but also in risk assessment, selection of treatment methods, and tracking its progress. Therefore, highly sensitive and selective detection and determination of cancer biomarkers are essential from the perspective of oncological diagnostics and planning the treatment process. Immunosensors are special types of biosensors that are based on the recognition of an analyte (antigen) by an antibody. Sandwich immunosensors apply two antibodies: a capture antibody and a detection antibody, with the antigen ‘sandwiched’ between them. Immunosensors’ advantages include not only high sensitivity and selectivity, but also flexible application and reusability. Surface-enhanced Raman spectroscopy, known also as the sensitive and selective method, uses the enhancement of light scattering by analyte molecules adsorbed on a nanostructured surface. The combination of immunosensors with the SERS technique further improves their analytical parameters. In this article, we followed the recent achievements in the field of sandwich SERS immunosensors for cancer biomarker detection and/or determination.

Immunoassay platform with surface-enhanced resonance Raman scattering for detecting trace levels of SARS-CoV-2 spike protein

The early diagnosis of Coronavirus disease (COVID-19) requires either an accurate detection of genetic material or a sensitive detection of viral proteins. In this work, we designed an immunoassay platform for detecting trace levels of SARS-CoV-2 spike (S) protein. It is based on surface-enhanced resonance Raman scattering (SERRS) of methylene blue (MB) adsorbed onto spherical gold nanoparticles (AuNPs) and coated with a 6 nm silica shell. The latter shell in the SERRS nanoprobe prevented aggregation and permitted functionalization with SARS-CoV-2 antibodies. Specificity of the immunoassay was achieved by combining this functionalization with antibody immobilization on the cover slides that served as the platform support. Different concentrations of SARS-CoV-2 antigen could be distinguished and the lack of influence of interferents was confirmed by treating SERRS data with the multidimensional projection technique Sammon's mapping. With SERRS using a laser line at 633 nm, the lowest concentration of spike protein detected was 10 pg/mL, achieving a limit of detection (LOD) of 0.046 ng/mL (0.60 pM). This value is comparable to the lowest concentrations in the plasma of COVID-19 patients at the onset of symptoms, thus indicating that the SERRS immunoassay platform may be employed for early diagnosis.

Exploring Sensitive Label-Free Multiplex Analysis with Raman-Coded Microbeads and SERS-Coded Reporters

Suspension microsphere immunoassays are rapidly gaining attention in multiplex bioassays. Accurate detection of multiple analytes from a single measurement is critical in modern bioanalysis, which always requires complex encoding systems. In this study, a novel bioassay with Raman-coded antibody supports (polymer microbeads with different Raman signatures) and surface-enhanced Raman scattering (SERS)-coded nanotags (organic thiols on a gold nanoparticle surface with different SERS signatures) was developed as a model fluorescent, label-free, bead-based multiplex immunoassay system. The developed homogeneous immunoassays included two surface-functionalized monodisperse Raman-coded microbeads of polystyrene and poly(4-tert-butylstyrene) as the immune solid supports, and two epitope modified nanotags (self-assembled 4-mercaptobenzoic acid or 3-mercaptopropionic acid on gold nanoparticles) as the SERS-coded reporters. Such multiplex Raman/SERS-based microsphere immunoassays could selectively identify specific paratope–epitope interactions from one mixture sample solution under a single laser illumination, and thus hold great promise in future suspension multiplex analysis for diverse biomedical applications.

From Raman to SESORRS: moving deeper into cancer detection and treatment monitoring

Raman spectroscopy is a non-invasive technique that allows specific chemical information to be obtained from various types of sample. The detailed molecular information that is present in Raman spectra permits monitoring of biochemical changes that occur in diseases, such as cancer, and can be used for the early detection and diagnosis of the disease, for monitoring treatment, and to distinguish between cancerous and non-cancerous biological samples. Several techniques have been developed to enhance the capabilities of Raman spectroscopy by improving detection sensitivity, reducing imaging times and increasing the potential applicability for in vivo analysis. The different Raman techniques each have their own advantages that can accommodate the alternative detection formats, allowing the techniques to be applied in several ways for the detection and diagnosis of cancer. This feature article discusses the various forms of Raman spectroscopy, how they have been applied for cancer detection, and the adaptation of the techniques towards their use for in vivo cancer detection and in clinical diagnostics. Despite the advances in Raman spectroscopy, the clinical application of the technique is still limited and certain challenges must be overcome to enable clinical translation. We provide an outlook on the future of the techniques in this area and what we believe is required to allow the potential of Raman spectroscopy to be achieved for clinical cancer diagnostics.

Nanostructure-based surface-enhanced Raman scattering for diagnosis of cancer

Cancer is a malignant disease that seriously affects human health and life. Early diagnosis and timely treatment can significantly improve the survival rate of cancer patients. Surface-enhanced Raman scattering (SERS) is an optical technology that can detect and image samples at the single-molecule level. It has the advantages of rapidity, high specificity, high sensitivity and no damage to the sample. The performance of SERS is highly dependent on the properties, size and morphology of the SERS substrate. Preparation of SERS substrates with good reproducibility and chemical stability is a key factor in realizing the wide application of SERS technology in cancer diagnosis. In this review we provide a detailed presentation of the latest research on SERS in cancer diagnosis and the detection of cancer biomarkers, mainly focusing on nanotechnological approaches in cancer diagnosis by using SERS. We also consider the future development of nanostructure-based SERS in cancer diagnosis.

Ultrasensitive, high-throughput and multiple cancer biomarkers simultaneous detection in serum based on graphene oxide quantum dots integrated microfluidic biosensing platform

Biomarkers play an important role in disease diagnosis and prognosis, which demand reliable, sensitive, rapid, and economic detection platform to conduct simultaneous multiple-biomarkers analysis in serum or body liquid. Here, we developed a universal biosensing platform through integrating the advantages of unique nanostructure and biochemistry properties of graphene oxide quantum dots and high throughput and low cost of microfluidic chip for reliable and simultaneous detection of multiple cancer antigen and antibody biomarkers. The performance of the proposed biosensing platform is validated through the representative cancer biomarkers including carcino-embryonic antigen (CEA), carbohydrate antigen 125 (CA125), α-fetoprotein (AFP), carbohydrate antigen 199 (CA199) and carbohydrate antigen 153 (CA153). It has a large linear quantification detection regime of 5-6 orders of magnitude and an ultralow detection limit of 1 pg/mL or 0.01 U/mL. Moreover, the proposed biosensing chip is capable of conducting 5-20 kinds of biomarkers from at least 60 persons simultaneously in 40 min with only 2 μL serum of each patient, which essentially reduces the detection cost and time to at least 1/60 of current popular methods. Clinical breast cancer and healthy samples detection results indicated its promising perspective in practical applications including cancer early diagnosis, prognosis, and disease pathogenesis study.

Ultrasensitive, high-throughput, and rapid simultaneous detection of SARS-CoV-2 antigens and IgG/IgM antibodies within 10 min through an immunoassay biochip

COVID-19 is now a severe threat to global health. Facing this pandemic, we developed a space-encoding microfluidic biochip for high-throughput, rapid, sensitive, simultaneous quantitative detection of SARS-CoV-2 antigen proteins and IgG/IgM antibodies in serum. The proposed immunoassay biochip integrates the advantages of graphene oxide quantum dots (GOQDs) and microfluidic chip and is capable of conducting multiple SARS-CoV-2 antigens or IgG/IgM antibodies of 60 serum samples simultaneously with only 2 μL sample volume of each patient. Fluorescence intensity of antigens and IgG antibody detection at emission wavelength of ~680 nm was used to quantify the target concentration at excitation wavelength of 632 nm, and emission wavelength of ~519 nm was used during the detection of IgM antibodies at excitation wavelength of 488 nm. The method developed has a large linear quantification detection regime of 5 orders of magnitude, an ultralow detection limit of ~0.3 pg/mL under optimized conditions, and less than 10-min qualitative detection time. The proposed biosensing platform will not only greatly facilitate the rapid diagnosis of COVID-19 patients, but also provide a valuable screening approach for infected patients, medical therapy, and vaccine recipients.

Real-Time Intraoperative Surface-Enhanced Raman Spectroscopy-Guided Thermosurgical Eradication of Residual Microtumors in Orthotopic Breast Cancer

Residual microtumors following surgical resection are the major cause for lethal cancer recurrence. However, it remains challenging to completely eliminate these residual microtumors. Here, we report an integrated strategy for image-guided surgical resection of tumors and intraoperative surface-enhanced Raman spectroscopy (SERS) guided thermosurgical elimination of residual microtumors using a "three-in-one" theranostic nanoprobe, termed the Au nanostar-based photoacoustic (PA), SERS, and thermosurgical (starPART) probe. This starPART probe, comprising an Au nanostar core, a Raman molecule layer, and a silica outer layer, draws upon the significant advantages of PA imaging, SERS detection, and photothermal tumor ablation. These prominent features enable preoperative PA imaging for surgical resection of tumors and intraoperative SERS-guided thermosurgery for complete elimination of residual microtumors. In vivo experiments confirm complete eradication of microtumors without local recurrence and with a 100% tumor-free survivability. This work therefore offers a robust platform for real-time intraoperative eradication of residual microtumors with significant improvement of surgical outcomes.

Interfacial interactions of SERS-active noble metal nanostructures with functional ligands for diagnostic analysis of protein cancer markers

Noble metal nanostructures with designed hot spots have been widely investigated as surface-enhanced Raman spectroscopy (SERS)-active substrates, particularly for selective and sensitive detection of protein cancer markers. For specific target recognition and efficient signal amplification, SERS probe design requires a choice of SERS-active nanostructures as well as their controlled functionalization with Raman dyes and target recognition entities such as antibodies. However, the chemical conjugation of antibodies and Raman dyes to SERS substrates has rarely been discussed to date, despite their substantial roles in detection schemes. The interfacial interactions of metal nanostructures with functional ligands during conjugation are known to be strongly influenced by the various chemical and physical properties of the ligands, such as size, molecular weight, surface charge, 3-dimensional structures, and hydrophilicity/hydrophobicity. In this review, we discuss recent developments in the design of SERS probes over the last 4 years, focusing on their conjugation chemistry for functionalization. A strong preference for covalent bonding is observed with Raman dyes having simpler molecular structures, whereas more complicated ones are non-covalently adsorbed. Antibodies are both covalently and non-covalently bonded to nanostructures, depending on their activity in the SERS probes. Considering that ligand conjugation is highly important for chemical stability, biocompatibility, and functionality of SERS probes, this review is expected to expand the understanding of their interfacial design, leading to SERS as one of the most promising spectroscopic analytical tools for the early detection of protein cancer markers.

The small silver nanoparticle-assisted homogeneous sensing of thiocyanate ions with an ultra-wide window based on surface-enhanced Raman-extinction spectroscopy

For the first time, we present an original sensing strategy with an ultra-wide detection window from 17 nM to 20 mM to detect SCN- ions. Initially, we investigated the clustering and optical properties of noble metal sol nanoparticles (NPs) due to the competitive interaction of thiocyanate ions (SCN-) and cetyltrimethylammonium bromide (CTAB) under weak acidic conditions, and found that different dimensions and scales of nanoclusters containing the alkyne-embedded Au@Ag NPs and relatively small Ag NPs could be achieved by the mediation of CTAB through electrostatic forces and hydrophobic interaction, in which SCN- could be covalently bonded with the silver surface of NPs to form a compact molecular layer (-Ag-S-C[triple bond, length as m-dash]N), and CTAB could only occupy remaining sites. In this process, we found that SCN- always runs counter to CTAB and tends to dissolve nanoclusters, so that they occupy the exposed surface of NPs in nanoclusters rather than the binding sites of one another. Remarkably, when the concentration of SCN- initially increased, two highly recognizable SERS emissions, which were assigned to alkyne reporter molecules (2208 cm-1) and C[triple bond, length as m-dash]N of SCN- (2110 cm-1), respectively, were rapidly detected, and their ratios (I2110/I2208) increased linearly proportional to the concentration of SCN- over a range of 17 nM to 172 μM, with a limit of detection (LOD) of 10 nM. With the further increase of SCN-, small Ag NPs started to desorb from the surface of individual Au@Ag NPs and dissociated in the solution but did not contribute to SERS signals. Instead, the surface plasmon resonance (SPR) peak of pure silver NPs at 385 nm increased gradually in the range from 0.5 to 20 mM with an LOD of 0.2 mM. Of particular significance, this simple sensor in conjunction with surface-enhanced Raman-extinction spectroscopy can be used for the rapid detection of extensive samples with an ultra-wide detection window, such as body fluids (saliva, urine, and serum) and food (milk powder and brassica vegetables), which is far superior to that of ion chromatography (IC).

A high-performance microfluidic detection platform to conduct a novel multiple-biomarker panel for ovarian cancer screening

Ovarian cancer is an important leading cause of cancer-related deaths among females, and a single biomarker does not have the sensitivity and specificity required for an effective ovarian cancer screening. Herein, we investigate a high-performance microfluidic detection platform to conduct a novel panel of multiple biomarkers for the early detection of ovarian carcinoma, which include CA125, HE4, OPN, MSLN, Hsp70, CA153, AFP, IL-6, and IL-8 using a microfluidic chip. High-throughput microfluidic chips and graphene oxide-assembled substrate are used to microprint repeatable capture antibody arrays and conduct multiple biomarkers in microscale volume samples. The proposed microfluidic platform achieves an ultralow detection limit of ∼1 pg mL⁻¹ and 0.01 U mL⁻¹ with excellent detection selectivity and a short detection time of 30 min. The analysis of serum biomarkers in 18 ovarian cancer patients and 4 healthy persons indicates a clear subgroup sorting between the high-grade serous ovarian carcinoma, borderline, and benign tumor patients, and healthy persons. The proposed detection platform and the biomarker panel are promising to conduct an early detection of ovarian cancer.

A high-performance microfluidic detection platform is developed to conduct a novel panel of multiple biomarkers for the early detection of ovarian carcinoma, which is promising for the early detection of ovarian cancer.

Surface-enhanced electrochemiluminescence combined with resonance energy transfer for sensitive carcinoembryonic antigen detection in exhaled breath condensates

The detection of biomarkers in exhaled breath condensates (EBCs) is regarded as a promising non-invasive diagnostic approach. However, the ultralow concentration of biomarkers in EBCs is a great challenge. Herein, a sensitive dual signal amplification strategy was developed based on surface-enhanced electrochemiluminescence (SEECL) combined with resonance energy transfer (RET). Gold nanoparticles-functionalized graphite-like carbon nitride nanohybrids (Au-g-C₃N₄ NHs) could be used as an energy transfer donor because of the good overlap between its emission peak and the absorption peak of tris(2,2'-bipyridine)ruthenium dichloride (Ru(bpy)₃Cl₂) at 460 nm. Gold-silicon dioxide core-shell nanoparticles doped with Ru(bpy)₃²⁺(Au@SiO₂-Ru) were employed as an energy transfer acceptor emitting at 620 nm. Moreover, the signals at 620 nm emitted by Ru (bpy)₃²⁺ were enhanced by 5 times, attributed to the localized surface plasmon resonance (LSPR) of gold nanoparticles (Au NPs). The detection of carcinoembryonic antigen (CEA) was performed by using two aptamers as the recognition unit; whereby aptamer 1 (Apt1) was modified on the surface of Au-g-C₃N₄ NHs, and aptamer 2 (Apt2) was banded on the surface of Au@SiO₂-Ru. In the presence of CEA, a sandwich structure was formed between Au-g-C₃N₄ NHs-Apt1-CEA and Apt2-Au@SiO₂-Ru, which resulted in an ultrasensitive detection of CEA. The proposed electrochemiluminescence sensor showed a wide linear relationship with the CEA concentration in the range from 1.0 pg mL^-1 to 5.0 ng mL^-1, with a limit of detection (LOD) of 0.3 pg mL^-1. Finally, the practicality of the proposed sensor was demonstrated to detect CEA in EBCs, and the obtained results were in good agreement with the enzyme-linked immunosorbent assay (ELISA) method.

A Fluorescence and Surface-Enhanced Raman Spectroscopic Dual-Modal Aptasensor for Sensitive Detection of Cyanotoxins

The ability to detect trace analytes without necessitating solid surface attachment or complicated processing steps would facilitate the translation of sensors for monitoring environmental toxins in the field. To address a critical unmet need in fresh water ecology, we have developed a dual-modal aptamer-based biosensor (aptasensor), featuring fluorescence and surface-enhanced Raman spectroscopy (SERS), for sensitive and selective detection of hepatotoxin microcystin-LR (MC-LR). The rational sensor design is based on the high affinity of the cyanine (Cy3) dye-modified complementary DNA (Cy3-cDNA) strand toward the plasmonic gold nanostars (GNSs) in comparison to the Cy3-cDNA/aptamer duplex. The preferential binding of MC-LR toward the MC-LR-specific aptamer triggers the dissociation of Cy3-cDNA/aptamer duplexes, which switches the Cy3's fluorescence "off" and SERS "on" due to the proximity of Cy3 dye to the GNS surface. Both fluorescence and SERS intensities are observed to vary linearly with the MC-LR concentration over the range of investigation. We have achieved high sensitivity and excellent specificity with the aptasensor toward MC-LR, which can be attributed to the fluorescence quenching effect, significant SERS enhancement by the GNSs, and the high affinity of the aptamer toward the MC-LR analytes. We further demonstrate the applicability of the present aptasensor for detection of MC-LR in a diverse set of real water samples with high accuracy and excellent reproducibility. With further refinement, we believe that the aptamer-driven complementary assembly of the SERS and fluorescence sensing constructs can be applied for rapid, multiplexed, and robust measurements of environmental toxins in the field.

Rapid, Label-free Optical Spectroscopy Platform for Diagnosis of Heparin-Induced Thrombocytopenia

The use of surface-enhanced Raman spectroscopy (SERS) to determine spectral markers for the diagnosis of heparin-induced thrombocytopenia (HIT), a difficult-to-diagnose immune-related complication that often leads to limb ischemia and thromboembolism, is proposed. The ability to produce distinct molecular signatures without the addition of labels enables unbiased inquiry and makes SERS an attractive complementary diagnostic tool. A capillary-tube-derived SERS platform offers ultrasensitive, label-free measurement as well as efficient handling of blood serum samples. This shows excellent reproducibility, long-term stability and provides an alternative diagnostic rubric for the determination of HIT by leveraging machine-learning-based classification of the spectroscopic data. We envision that a portable Raman instrument could be combined with the capillary-tube-based SERS analytical tool for diagnosis of HIT in the clinical laboratory, without perturbing the existing diagnostic workflow.

Gap-enhanced Raman tags: fabrication, optical properties, and theranostic applications

Gap-enhanced Raman tags (GERTs) are emerging probes of surface-enhanced Raman scattering (SERS) spectroscopy that have found promising analytical, bioimaging, and theranostic applications. Because of their internal location, Raman reporter molecules are protected from unwanted external environments and particle aggregation and demonstrate superior SERS responses owing to the strongly enhanced electromagnetic fields in the gaps between metal core-shell structures. In this review, we discuss recent progress in the synthesis, simulation, and experimental studies of the optical properties and biomedical applications of novel spherically symmetrical and anisotropic GERTs fabricated with common plasmonic metals—gold (Au) and silver (Ag). Our discussion is focused on the design and synthetic strategies that ensure the optimal parameters and highest enhancement factors of GERTs for sensing and theranostics. In particular, we consider various core-shell structures with build-in nanogaps to explain why they would benefit the plasmonic GERTs as a superior SERS tag and how this would help future research in clinical analytics and therapeutics.

Surface modification of plasmonic noble metal-metal oxide core-shell nanoparticles

A comprehensive survey on the methods for the surface modification of plasmonic noble metal-metal oxide core-shell nanoparticles is presented. The review highlights various strategies for covalent attachment and electrostatic binding of molecules and molecular ions to core-shell nanoparticles with a focus on plasmonically active silver and gold nanoparticles encapsulated by SiO₂ and TiO₂ shells.

Engineering State-of-the-Art Plasmonic Nanomaterials for SERS-Based Clinical Liquid Biopsy Applications

Precision oncology, defined as the use of the molecular understanding of cancer to implement personalized patient treatment, is currently at the heart of revolutionizing oncology practice. Due to the need for repeated molecular tumor analyses in facilitating precision oncology, liquid biopsies, which involve the detection of noninvasive cancer biomarkers in circulation, may be a critical key. Yet, existing liquid biopsy analysis technologies are still undergoing an evolution to address the challenges of analyzing trace quantities of circulating tumor biomarkers reliably and cost effectively. Consequently, the recent emergence of cutting-edge plasmonic nanomaterials represents a paradigm shift in harnessing the unique merits of surface-enhanced Raman scattering (SERS) biosensing platforms for clinical liquid biopsy applications. Herein, an expansive review on the design/synthesis of a new generation of diverse plasmonic nanomaterials, and an updated evaluation of their demonstrated SERS-based uses in liquid biopsies, such as circulating tumor cells, tumor-derived extracellular vesicles, as well as circulating cancer proteins, and tumor nucleic acids is presented. Existing challenges impeding the clinical translation of plasmonic nanomaterials for SERS-based liquid biopsy applications are also identified, and outlooks and insights into advancing this rapidly growing field for practical patient use are provided.

A magnetic SERS immunosensor for highly sensitive and selective detection of human carboxylesterase 1 in human serum samples

Hepatocellular carcinoma (HCC) is a common and lethal cancer. New serum markers for detecting HCC are urgently needed. Human carboxylesterase 1 (hCE1) is an important member of the serine hydrolase superfamily and is closely related to the occurrence of HCC. It can be used as a good serum marker for early diagnosis of HCC. Here, we developed a surface enhanced Raman scattering (SERS)- based magnetic immunosensor that specifically recognizes and detects trace amounts of hCE1 in human serum via a sandwich structure consisting of a SERS tags, magnetic supporting substrates, and target antigen (hCE1). The SERS tags are 4-mercaptobenzoic acid (4-MBA)-labeled AgNPs, and the SERS supporting substrates are composed of a raspberry-like morphology of Fe₃O₄@SiO₂@AgNPs magnetic nanocomposites surface-functionalized with a hCE1 antibody. The prepared SERS magnetic immunosensor exhibits excellent selectivity and extremely high sensitivity for hCE1 detection. The SERS signal and logarithm of hCE1 concentration presented a wide linear response range of 0.1 ng mL^-1 to 1.0 mg mL^-1, and the detection limit of hCE1 was 0.1 ng mL^-1. The results indicate that the immunosensor can be used for the rapid determination of hCE1 in human serum without a complicated sample pre-treatment. Furthermore, the immunosensor has good reproducibility and stability, and has a promising prospect for the quantitative detection of other tumor markers in early clinical diagnosis.

Combining Experiments and Theoretical Modeling To Interrogate the Anisotropic Growth and Structure-Plasmonic Property Relationships of Gold Nanostars

We present a combined strategy of experiments and theoretical modeling for understanding the evolution of the morphology and plasmonic properties of gold nanostars (GNSs) in the seed-mediated synthesis by changing the poly(vinylpyrrolidone) (PVP) molecular weight, PVP concentration, and synthesis temperature. A dramatic change of the morphology of GNSs as a function of these synthesis parameters is observed that is related to variations of the plasmonic properties and thus surface-enhanced Raman spectroscopy (SERS) enhancement. We observe the favorable growth of anisotropic GNS structures with sharp protruding tips using PVP of low molecular weight and of rounded GNSs with short protruding tips using PVP of high molecular weight. The PVP concentration has less influence on the core size than on the tip length of GNSs. The high synthesis temperature causes the rounding of the GNS structure. Finite-difference time-domain (FDTD) simulations reveal a remarkable correlation of the GNS morphology with the plasmonic properties as well as the SERS enhancement. The maximum local electric field enhancement occurs at the apex of the sharp protruding tips of the GNSs. The weak plasmonic coupling is observed between the protruding tips of GNSs because of their large separation distance, and increasing the number of protruding tips beyond two only increases the extinction cross section without further red-shifting the plasmon peak. A resonance overlap of the plasmon band with the incident laser wavelength is responsible for the morphology-dependent plasmonic properties and SERS enhancement. The present work demonstrates that a mechanistic understanding of the structural evolution of GNSs along with their morphology-plasmonic property correlation can be achieved through the combination of experimental investigations and FDTD-based theoretical modeling.

Ultrasensitive Detection of Hepatotoxic Microcystin Production from Cyanobacteria Using Surface-Enhanced Raman Scattering Immunosensor

Microcystin-LR (MC-LR) is considered the most common hazardous toxin produced during harmful algal blooms. In addition to potential risk of long-term exposure to low concentrations in drinking water, acute toxicity due to MC-LR resulting from algal blooms could result in fatalities in rare cases. Although several methods are currently available to detect MC-LR, development of a low-cost, ultrasensitive measurement method would help limit exposure by enabling early detection and continuous monitoring of MC-LR. Here, we develop a surface-enhanced Raman scattering (SERS) spectroscopic immunosensor for detection and quantification of the hepatotoxic MC-LR toxin in aquatic settings with excellent robustness, selectivity, and sensitivity. We demonstrate that the developed SERS sensor can reach a limit of detection (0.014 μg/L) at least 1 order of magnitude lower and display a linear dynamic detection range (0.01 μg/L to 100 μg/L) 2 orders of magnitude wider in comparison to the commercial enzyme-linked immunosorbent assay test. The superior analytical performance of this SERS immunosensor enables monitoring of the dynamic production of MC-LR from a Microcystis aeruginosa culture. We believe that the present method could serve as a useful tool for detection of hepatotoxic microcystin toxins in various aquatic settings such as drinking water, lakes, and reservoirs. Further development of this technique could result in single-cell microcystin resolution or real-time monitoring to mitigate the associated toxicity and economic loss.

Surface-enhanced Raman spectroscopy (SERS) nanoprobes for ratiometric detection of cancer cells

A surface-enhanced Raman spectroscopic strategy is developed for ratiometric detection of cancer cells by quantifying the expression ratio of extracellular biomarkers.

Enhanced immunofluorescence detection of a protein marker using a PAA modified ZnO nanorod array-based microfluidic device

Zinc oxide (ZnO) often serves as protein microarray substrates owing to its outstanding fluorescence enhancement effect. However, the integration of functional substrates with microfluidic technology to detect cancer biomarkers still needs to be optimized and promoted, for example, the optimization of micro/nanostructure and hydrophilic modification strategies for fluorescence immunoassays. Here, ZnO nanorod arrays were constructed on the inner wall of glass capillaries through a microfluidic chemical method, and the electrostatic layer by layer self-assembly was applied to modify the nanorod array with hydrophilic polyelectrolyte-polyacrylic acid (PAA). The effects of the flow rate and the reagent concentration on the morphology of the ZnO nanorod array were investigated. The ZnO nanorod array-based glass capillary, prepared at 25 μL min-1 for 4 min with 50 mM Zn2+ in solution, showed a remarkable enhancement in fluorescence performance. In addition, the introduction of PAA suppressed the interference of nonspecific protein and improved the antibody loading capacity effectively. In the detection of carcinoembryonic antigen, the limit of detection reached 100 fg mL-1, which indicated that the ZnO@PAA nanorod array-based microfluidic device exhibits remarkable fluorescence detection performance towards protein markers and possesses potential to be applied to point-of-care diagnostics and high throughput cancer biomarker detection.

SERS Sensors: Recent Developments and a Generalized Classification Scheme Based on the Signal Origin

Owing to its extreme sensitivity and easy execution, surface-enhanced Raman spectroscopy (SERS) now finds application for a wide variety of problems requiring sensitive and targeted analyte detection. This widespread application has prompted a proliferation of different SERS-based sensors, suggesting the need for a framework to classify existing methods and guide the development of new techniques. After a brief discussion of the general SERS modalities, we classify SERS-based sensors according the origin of the signal. Three major categories emerge from this analysis: surface-affinity strategy, SERS-tag strategy, and probe-mediated strategy. For each case, we describe the mechanism of action, give selected examples, and point out general misconceptions to aid the construction of new devices. We hope this review serves as a useful tutorial guide and helps readers to better classify and design practical and effective SERS-based sensors.

An ultrasensitive polydopamine bi-functionalized SERS immunoassay for exosome-based diagnosis and classification of pancreatic cancer

Early diagnosis and metastasis monitoring for pancreatic cancer are extremely difficult due to a lack of sensitive liquid biopsy methods and reliable biomarkers. Herein, we developed easy-to-prepare and effective polydopamine-modified immunocapture substrates and an ultrathin polydopamine-encapsulated antibody-reporter-Ag(shell)-Au(core) multilayer (PEARL) Surface-Enhanced Raman Scattering (SERS) nano-tag with a quantitative signal of the Raman reporter at 1072 cm^-1, which achieved ultrasensitive and specific detection of pancreatic cancer-derived exosomes with a detection limit of only one exosome in 2 μL of sample solution (approximately 9 × 10^-19 mol L^-1). Furthermore, by analyzing a 2 μL clinical serum sample, the migration inhibitory factor (MIF) antibody-based SERS immunoassay could not only discriminate pancreatic cancer patients (n = 71) from healthy individuals (n = 32), but also distinguish metastasized tumors from metastasis-free tumors, and Tumor Node Metastasis (TNM) P1-2 stages from the P3 stage (the discriminatory sensitivity was 95.7%). Thus, this novel immunoassay provides a powerful tool for the early diagnosis, classification and metastasis monitoring of pancreatic cancer patients.

New trends in plasmonic (bio)sensing

The strong enhancement and localization of electromagnetic field in plasmonic systems have found applications in many areas, which include sensing and biosensing. In this paper, an overview will be provided of the use of plasmonic phenomena in sensors and biosensors with emphasis on two main topics. The first is related to possible ways to enhance the performance of sensors and biosensors based on surface plasmon resonance (SPR), where examples are given of functionalized magnetic nanoparticles, magnetoplasmonic effects and use of metamaterials for SPR sensing. The other topic is focused on surface-enhanced Raman scattering (SERS) for sensing, for which uniform, flexible, and reproducible SERS substrates have been produced. With such recent developments, there is the prospect of improving sensitivity and lowering the limit of detection in order to overcome the limitations inherent in ultrasensitive detection of chemical and biological analytes, especially at single molecule levels.