key: cord-0834340-ah692ift
authors: Bistaffa, Maria J.; Camacho, Sabrina A.; Pazin, Wallance M.; Constantino, Carlos J.L.; Oliveira, Osvaldo N.; Aoki, Pedro H.B.
title: Immunoassay platform with surface-enhanced resonance Raman scattering for detecting trace levels of SARS-CoV-2 spike protein
date: 2022-03-17
journal: Talanta
DOI: 10.1016/j.talanta.2022.123381
sha: 3efd70297a77401eeebcce50cd5b5f8c47bed222
doc_id: 834340
cord_uid: ah692ift

The early diagnosis of 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.

Early diagnosis of the Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) [1] , [2] , [3] is efficient to prevent the fast spreading of the virus by inhibiting human-human transmission through direct routes [4] [5] [6] . The most used detection method has been reverse transcription-polymerase chain reaction (RT-PCR). The test consists in converting the viral mRNA into DNA and applying PCR reactions to DNA amplification and detection [7, 8] . RT-PCR is highly specific for SARS-CoV-2, but its accuracy may be compromised by variations in sample collection and persistence of viral RNA in the nasal cavity/throat, thus leading to false-negative/false-positive results [9] [10] [11] [12] . Additionally, PCR involves multiple sample processing steps (e.g., nucleic acid extraction, purification, and amplification), making it time-consuming and expensive [13] . The other types of widely used tests are serological tests [14] [15] [16] , which are more stable than viral RNA tests owing to the uniform distribution of proteins in the blood, tending to reduce falsenegatives [17] . However, these tests to detect SARS-CoV-2 antibodies via enzyme-linked immunosorbent assays (ELISA) [12, [18] [19] [20] or lateral flow immunoassays (LFAs) [21] cannot be used for early diagnosis. Indeed, it can take 2-3 weeks for viral-specific antibodies to be produced after infection.

Biosensors with distinct architectures have been developed for early detection of Covid-19, with the spike protein being the most used biomolecule for diagnosis. For instance, Soares et al. [22] developed an immunosensor made of carboxymethyl chitosan coated with an active layer of specific antibodies, capable to detect the spike protein via electrical impedance spectroscopy. The spike protein could also be detected in sensing chips containing metamaterials and using terahertz time-domain spectroscopy [23] , and with magnetic microrobots immuno-sandwich assays [24] . For diagnosis of Covid-19 through detection of nucleocapsid protein, Chen et al. [25] constructed an electrical J o u r n a l P r e -p r o o f double layer biosensor, while Qi et al. [26] designed a fast-responding aptasensor based on a low-cost microelectrode array chip. RNA fragments have also been applied in detecting Covid-19 with a one-pot loop probe-mediated isothermal amplification method [27] . There are serological tests which can be used for early diagnosis, for they can detect viral proteins at the onset of symptoms. For instance, SARS-CoV-2 spike (S1) protein was detected in the plasma of Covid-19 patients at concentrations ranging from ∼8 to 20,000 pg/mL [28] , thus permitting an accurate and early detection of Covid-19. One important requirement to achieve high accuracy is the capability to detect trace levels of such proteins. This motivates the search for highly sensitive methods, such as surfaceenhanced resonance Raman scattering (SERRS) to detect the target molecule adsorbed on plasmonic metallic nanostructures [29, 30] . SERRS has been used to detect trace level concentrations of neurotransmitters [31] , proteins [32] [33] [34] , pesticides [35] [36] [37] , heavy metal ions [38] , and cancer biomarkers [33, 34, 39] . Combining SERRS with an immunoassay platform may allow one to detect different proteins and increase sensitivity [40, 41] .

In this paper, we report on an immunoassay platform for early diagnosis of Covid-19 with SARS-CoV-2 spike (S1) protein (antigen) used as target molecule. Gold nanoparticles (AuNPs) were conjugated with methylene blue (MB) enclosed by a silica layer to avoid uncontrolled aggregation and chemical degradation. MB is a Raman reporter molecule, whose absorption is in resonance with the excitation laser line (at 633 nm), with which one may generate surface-enhanced resonance Raman scattering (SERRS). The specificity to the immunoassay platform was warranted by functionalizing the nanoparticle (SERRS nanoprobe) with SARS-CoV-2 antibodies. These Ab-SARS-CoV-2-SERRS nanoprobes can detect concentrations of SARS-CoV-2 spike protein down to 10 pg/mL. J o u r n a l P r e -p r o o f

Spherical gold nanoparticles (AuNPs) with ca. 14  3 nm diameter were synthesized via citrate reduction [42] . The procedure consisted in preparing 150 x 10 -3 L tetrachloroauric acid (HAuCl4 0. 

The SERRS nanoprobes were functionalized with SARS-CoV-2 antibodies by adapting established protocols [47, 48] . of SERRS nanoprobes with SARS-CoV-2 antibodies is also illustrated in Scheme 1.

J o u r n a l P r e -p r o o f SARS-CoV-2 Spike glycoprotein S1 antibodies were functionalized on the surface of cover slides following published protocols [43, 47, 48] . This antibody immobilization onto the cover slides is essential to ensure the effectiveness of the immunoassay platform. For the specific antibody-antigen interaction prevents the antigens to be added in the next step from being removed in the rinsing step with PBS solution. The cover slides were cleaned with milli-Q water and ethanol in an ultrasonic 

The concentrations of recombinant human coronavirus SARS-CoV-2 Spike glycoprotein S1 (SARS-CoV-2 antigen; ref: ab273068, ABCam) were chosen based on the clinical appeal for Covid-19 diagnosis [49, 50] , from 1 x 10 -11 g/mL to 1 x 10 -4 g/mL. 

The resonance Raman and SERRS spectra were acquired using an in-Via Raman microscope (Renishaw Inc., Hoffman Estates, IL) equipped with a 633 nm laser line, 1800 lines/mm diffraction grating, 10 s acquisition time, laser power attenuated to 10%, and a 50× (NA = 0.75) dry objective (Leica Microsystems). SERRS mapping was carried out by selecting a 40 x 40 µm² area with 3 µm step, thus obtaining a total of 196 spectra for each mapped concentration. The baseline correction and analysis of Raman spectra was performed using Origin Pro V8.5 software (OriginLab Corporation, Northants). The data were also evaluated using the multidimensional projection technique referred to as Sammon's mapping [51, 52] . With this methodology data from a multidimensional space are projected onto a 2D space trying to preserve similarity relationships within the original data set. Scheme 1. Illustration of the design of Ab-SARS-CoV-2-SERRS nanoprobes: (i) AuNP (ca. 14 nm diameter) prepared via citrate reduction; (ii) AuNP bonded with MB molecules, whose 3D molecular structure is indicated by the arrow; (iii) AuNP + MB molecules + an ultrathin silica shell (SERRS nanoprobeca. 6 nm shell thickness) where the amino group of APTMS is firstly bonded to the negative charge surrounding the AuNP followed by sodium silicate binding with APTMS, forming the 3D complex molecular structure indicated by the arrow; (iv) activation of carboxyl terminal groups;

(v) functionalization with Ab-SARS-CoV-2 (Ab-SARS-CoV-2-SERRS nanoprobe (ca. 31 nm); (vi) functionalization of the cover slide with SARS-CoV-2 antibodies; (vii) incubation of SARS-CoV-2 antigen onto the cover slide already functionalized with SARS-CoV-2 antibodies in the previous step; and (viii) incubation with Ab-SARS-CoV-2-SERRS nanoprobes completing the SERRS immunoassay platform, followed by Raman measurement.

As depicted in the schematic illustration of the Ab-SARS-CoV-2-SERRS nanoprobe in Scheme 1, methylene blue (MB) molecules are embedded between spherical gold nanoparticles (AuNPs) and ultrathin silica shells via covalent bond of N-CH3 groups J o u r n a l P r e -p r o o f from MB with AuNPs, followed by functionalization with SARS-CoV-2 antibodies. The silica coating precludes aggregation and the leaching of MB during processing and assay operations. Furthermore, it provides an outstanding surface functionalization to render molecular specificity to the nanoprobes [43] . The UV-Vis absorption spectrum of the MB solution at 10 -5 mol/L and the extinction spectra of AuNPs and SERRS nanoprobes are shown in Figure 1(a) . Two absorption peaks at 613 and 664 nm in the MB spectrum are assigned to n-π* transitions (n is the free doublet on the nitrogen atom of C=N bond and free doublet of S atom on S=C bond) [53] . The extinction of the localized surface plasmon resonance (LSPR) shifted from 518 nm (AuNPs) to 523 nm (SERRS nanoprobes) owing to the conjugated MB and silica shell covering the AuNP [43, 47, 48] . The zeta potential and diameters obtained from DLS for AuNPs, SERRS nanoprobes and Ab-SARS-CoV-2-SERRS nanoprobes are displayed in Figure 1(b) . AuNPs have approximately 14 ± 3 nm diameter, which increased to 20 ± 6 nm with the MB conjugation + silica coating and to 31 ± 13 nm with the functionalization of SARS-CoV-2 antibodies. The zeta potential was also affected, especially from the AuNPs (-32.4 ± 0.6 mV) to the SERRS nanoprobes (-18 ± 2 mV), whose increase is related to the reduction of negative charges around the nanoparticles. The AuNPs synthesized via citrate are negatively charged [42] and the cationic MB [54] and inert silica shell [55, 56] decrease the surface charge. After functionalization with the antibodies a slight decrease is noted in zeta potential (-22 ± 1 mV). The prevalence of disulfide bonds and carboxyl-terminal groups in relation to amino-terminal groups in the antibody might lead to an increase of negative charges [57] . In fact, the MB molecules adsorbed on AuNPs might undergo fluorescence quenching [58] while forming a protective barrier that avoids the direct contact between the outermost MB molecules and the AuNPs surface. This protective barrier made of MB molecules provides a metal-dye distance that allows a continuous transition from J o u r n a l P r e -p r o o f fluorescence quenching to fluorescence enhancement [59, 60] . On the other hand, in the SERRS nanoprobes spectrum (gray) the Raman fingerprint from MB is clear [54, 61] , with a slightly modified spectral profile compared with the resonance Raman spectrum of MB solution (blue). These spectral changes might be induced by MB-AuNP binding and subsequent silica shell enclosure, which did not affect the chemical structure of MB.

In fact, the SERRS activity of antigen-Ab-SARS-CoV-2-SERRS nanoprobes (green) also demonstrated that MB molecules did not undergo any chemical modification during building of the immunoassay platform and/or with the tests in SERRS detection. The contact between the metallic surface and MB molecules leads to fluorescence quenching, favoring the enhanced Raman signal [62] . Additionally, the spectrum (green) for Ab-

nanoprobes display slight changes in intensity and band position, with spectral profiles comparable to the resonant Raman spectrum of MB solution (blue; Figure 2 ).

The relative intensity of the bands from vibrational modes of N-CH3 symmetrical stretching and C-C ring stretching is inverted when the resonant Raman spectrum of MB solution is compared with the SERRS spectra of the SERRS nanoprobes and of the antigen-Ab-SARS-CoV-2-SERRS nanoprobes. In MB solution the band at 1623 cm -1 assigned to C-C ring stretching is the most intense with the band at 1396 cm -1 assigned to the vibrational mode N-CH3 symmetrical stretching being the second most intense. In the SERRS spectra of SERRS nanoprobes and antigen-Ab-SARS-CoV-2-SERRS nanoprobes, the band from N-CH3 symmetrical stretching (at 1383 cm -1 ) is the most intense, followed by the band from C-C ring stretching [54] . This might suggest a preferential orientation of MB molecules on the nanoprobes. The enhanced Raman signal J o u r n a l P r e -p r o o f is highly dependent on the distance between the adsorbed molecule and metallic surface, and on molecular orientation [63] [64] [65] [66] . Therefore, it can be assumed that MB molecules are adsorbed on the AuNP surface through the N-CH3 bond, in a more inclined orientation to the vertical line. The dipole of C-C stretching ring is parallel to ⃗ local surrounding the metallic nanoparticle, being perpendicular to its surface. This molecular orientation of MB leads to a maximum enhancement of the 1383 cm -1 band (N-CH3 symmetrical stretching) followed by the band at 1623 cm -1 (C-C ring stretching), respectively. A proposal for the orientation of MB molecules on the AuNP is given in Figure S1 (Supplementary Material). Antigen-Ab-SERRS nanoprobe [54, 67] ν (C-C) ring  1623  1623  1616  1583  1502 1502 [67] νasym(C-N) 1438 1440 [54, 68] νsym N-CH3 1396 1382 1383 [69] −CH2 twist 1316 1312 [68] α(C-H) 1301 1273 1241 1237 [67] ν(C-N) 1180 [70] C-C (vibration) 1155 1158 [67] γ(C-H) 1127 1127 [54, 67] β Each SERRS mapping was plotted by taking the band intensity at 1383 cm -1 (peak intensity without any baseline correction) (Figure 3) , where the brighter spots refer to higher intensities. This band was chosen owing to its high intensity; it is assigned to the vibrational mode of N-CH3 symmetrical stretching, which is covalently bonded to Au surface (Figure 3(k) ). The distribution of SERRS intensities indicates that the nanoprobes are fairly dispersed on the platform surface. SARS-CoV-2 antigen was detected at every probed concentration (Figure 3(a)-(h) ). The lowest concentration of SARS-CoV-2 antigen was 0.01 ng/mL (0.13 pM), smaller than the concentrations reported with recent methodologies to detect Covid-19. Indeed, the detection limits were 5 ng/mL for lateral flow via ACE2 receptor [72] , 12.6 nM for single-walled carbon nanotube (SWCNT)based fluorescence sensing [73] , 50 pg/mL for microfluidic magneto immunosensor using electrochemical measurements [13] , and 0.22 pM for plasmonic photothermal biosensors [74] .

J o u r n a l P r e -p r o o f The SERRS intensity at 1383 cm -1 was averaged from a total of 196 SERRS spectra recorded in triplicate for each concentration of SARS-CoV-2 antigen, from which the adsorption curve in Figure 4a was obtained. The isotherm could be fitted with the Langmuir adsorption model [75] [76] [77] for the range of detection analyzed here, from 100 µg/mL to 0.01 ng/mL of SARS-CoV-2 spike (S1) glycoprotein (coefficient of determination, R 2 = 0.9604). At low concentrations (from 100 ng/mL to 0.01 ng/mL) the increase in SERRS intensity with the SARS-CoV-2 antigen concentration could be J o u r n a l P r e -p r o o f approximated with a linear function, as shown in Figure 4b (coefficient of determination, R 2 = 0.9741). At this linear regime, no intermolecular interactions are predicted to occur and a calibration curve was used to determine the limit of detection (LOD) of the SERRS immunoassay platform [76, 78] . LOD was calculated according to the following equation [48, 77, 79, 80] :

where SDBlank is the standard deviation of the "Blank" sample and b is the slope of the calibration curve. The LOD (0.046 ng/mL or 0.60 pM) obtained here is compared in Table   2 with published research results of detection of SARS-CoV-2 spike (S1) glycoprotein.

For instance, our SERRS immunoassay platform was capable to detect the antigen at lower concentrations than with enzyme-linked immunoassay (ELISA) for spike (S1) protein in recombinant SARS-CoV-2 using electrochemical techniques, which was 20 ng/mL [49] . The detection of SARS-CoV-2 antigen at picomolar concentration is possible due to the enhancement in MB resonance Raman signal enabled by SERRS nanoprobes.

This sensitivity permits detection of viral proteins at the onset of symptoms, as expected from the ca. 8 pg/mL concentration of SARS-CoV-2 spike (S1) protein in the plasma of Covid-19 patients [28] . Therefore, the sensitivity, photostability and rapid accuracy of the SERRS immunoassay designed here confirm the potential for detecting low SARS-CoV-2 antigen concentrations, relevant for early diagnosis. In order to make the analysis of SERRS mapping more quantitative, we employed the multidimensional projection technique referred to as Sammon's mapping [51, 52] . In this technique, the data from a high-dimensional space are mapped onto a lower dimension space where the similarity relationships among data instances in the highdimensional space should be preserved [52] . Each circle in Figure 5 represents an average of the whole 196 spectra of the SERRS area mapping displayed in Figure 3 concentrations (ng/mL)), with exception of the concentration at 1 µg/mL. Even within these groups, the concentrations are relatively apart from each other, confirming the ability of the multidimensional projection in distinguishing the SERRS mappings of different concentrations. Besides, the separation from the controls "no antigen" and "blank" group is increased with increasing antigen concentration from 0.01 ng/mL to 100 µg/mL, confirming the distinguishing ability of the immunoassay platform. In particular, the "no antigen" and "blank" group are closer to the group of lowest concentrations (0.1 and 0.01 ng/mL), comparable to that in Figure 3 .

We also performed interferents analysis using the pharmaceutical drugs:

paracetamol, azithromycin, dexamethasone and ivermectin, which have been used against Covid-19, though considered ineffective. The Sammon's mapping data plotted in Figure   5 includes the different concentrations of SARS-CoV-2 antigen and the 4 interferents (separately and mixed). The different concentrations of SARS-CoV-2 antigen are easily distinguishable from each other and positioned apart from the data of the interferents.

They are grouped on the bottom right of the Sammon's mapping, highlighting the ability of our SERRS immunoassay platform to also distinguish interferents. Soares et al. [22] developed a biosensor capable to differentiate various concentrations of SARS-CoV-2 antigen from control experiments (interferents) applying IDMAP technique to analyze electrical impedance spectroscopy measurements. Moreover, SARS-CoV-2 antigens embedded into to the interferents could also be discriminated, as shown in Figure S2 . for the interferents paracetamol, azithromycin, dexamethasone, ivermectin and mixed interferents. The controls "No antigen" and "Blank" were also evaluated. Each circle represents the average of the 196 SERRS spectra for each mapping of a given antigen concentration, without any spectral treatment. The proximity of the circles indicates the similarity between the data (i.e., similar the SERRS mappings responses will lead to circles closer to each other).

J o u r n a l P r e -p r o o f

The surface-enhanced resonance Raman scattering (SERRS) immunoassay platform designed here was able to detect trace levels of viral spike (S1) protein from SARS-CoV- e.g. with regard to the time taken for the test. As it is now, the test takes at least 2 h and requires a rather sophisticated instrument (Raman spectrometer). However, this time may be shortened with optimization procedures which could also enhance sensitivity.

Furthermore, it is possible to employ portable Raman spectrometers, and this would decrease the cost of the tests considerably.

J o u r n a l P r e -p r o o f

Worldometers: Coronavirus update (live) cases and deaths from Covid-19 virus pandemic

World Health Organization

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and coronavirus disease-2019 (Covid-19): The epidemic and the challenges

A nanomechanical study on deciphering the stickiness of SARS-CoV-2 on inanimate surfaces

Projecting the transmission dynamics of SARS-CoV-2 through the postpandemic period

On the challenges for the diagnosis of SARS-CoV-2 based on a review of current methodologies

RT-qPCR Testing of SARS-CoV-2: A Primer

Covid-19 testing: the threat of falsenegative results

Virological assessment of hospitalized patients with Covid-19

Sensitivity of chest CT for Covid-19: comparison to RT-PCR

Molecular and serological investigation of 2019-nCoV infected patients: implication of multiple shedding routes

Microfluidic magneto immunosensor for rapid, high sensitivity measurements of SARS-CoV-2 nucleocapsid protein in serum

Multiplexed, quantitative serological profiling of Covid-19 from blood by a point-of-care test

Development and evaluation of a serological test for diagnosis of Covid-19 with selected recombinant spike proteins

Clinical and Analytical Performance of an Automated Serological Test That Identifies S1/S2-Neutralizing IgG in Covid-19 Patients Semiquantitatively

Evolving status of the 2019 novel coronavirus infection: Proposal of conventional serologic assays for disease diagnosis and infection monitoring

Profiling early humoral response to diagnose novel coronavirus disease (Covid-19)

Evaluation of nucleocapsid and spike protein-based enzyme-linked immunosorbent assays for detecting antibodies against SARS-CoV-2

Antibody responses to SARS-CoV-2 in patients with novel coronavirus disease 2019

Development and clinical application of a rapid IgM-IgG combined antibody test for SARS-CoV-2 infection diagnosis

Diagnostics of SARS-CoV-2 infection using electrical impedance spectroscopy with an immunosensor to detect the spike protein

Detection and discrimination of SARS-CoV-2 spike protein-derived peptides using THz metamaterials

Collective behavior of magnetic microrobots through immuno-sandwich assay: On-the-fly Covid-19 sensing

Salivabased Covid-19 detection: A rapid antigen test of SARS-CoV-2 nucleocapsid protein using an electrical-double-layer gated field-effect transistor-based biosensing system

Capacitive aptasensor coupled with microfluidic enrichment for real-time detection of trace SARS-CoV-2 nucleocapsid protein

Rapid and highly specific detection of communicable pathogens using one-pot loop probe-mediated isothermal amplification (oLAMP)

Ultra-sensitive serial profiling of SARS-CoV-2 antigens and antibodies in plasma to understand disease progression in Covid-19 patients with severe disease

Recent advances on the development of plasmon-assisted biosensors for detection of C-reactive protein

New trends in plasmonic (Bio)sensing

SERS of Epinephrine: A computational and experimental study

Integration of a fiber-based cell culture and biosensing system for monitoring of multiple protein markers secreted from stem cells

A microsphere nanoparticle based-serum albumin targeted adsorption coupled with surfaceenhanced Raman scattering for breast cancer detection

Label-free surface-enhanced Raman spectroscopy for diagnosis and analysis of serum samples with different types lung cancer

Surface-enhanced Raman scattering of flexible cotton fiber-Ag for rapid adsorption and detection of malachite green in fish

Investigation of pesticide residue removal effect of gelatinized starch using surface-enhanced Raman scattering mapping

Reliable SERS detection of pesticides with a large-scale selfassembled Au@4-MBA@Ag nanoparticle array

Silver enriched silver phosphate microcubes as an efficient recyclable SERS substrate for the detection of heavy metal ions

Label-free diagnosis of breast cancer based on serum protein purification assisted surface-enhanced Raman spectroscopy

Ultrasensitive detection of SARS-CoV-2 spike protein in untreated saliva using SERS-based biosensor

AuNP array coated substrate for sensitive and homogeneous SERS-immunoassay detection of human immunoglobulin G

A study of the nucleation and growth processes in the synthesis of colloidal gold

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

Ultrasensitive, multiplex Raman frequency shift immunoassay of liver cancer biomarkers in physiological media

Plasmonenhanced fluorescence and spectral modification in SHINEF

Surface analysis using shell-isolated nanoparticle-enhanced Raman spectroscopy

Immunoassay quantification using surface-enhanced fluorescence (SEF) tags

Zika immunoassay based on surface-enhanced Raman scattering nanoprobes

Rapid SARS-CoV-2 detection using electrochemical immunosensor

Functionalized terahertz plasmonic metasensors: Femtomolar-level detection of SARS-CoV-2 spike proteins

Content-based text mapping using multi-dimensional projections for exploration of document collections

A nonlinear mapping for data structure analysis

Aggregation of methylene blue in frozen aqueous solutions studied by absorption spectroscopy

SERS signal enhancement of methylene blue-embedded agglomerated gold nanorod@SiO2 core@shell composites

Molecular-level effects on cell membrane models to explain the phototoxicity of gold shell-isolated nanoparticles to cancer cells

The efficiency of photothermal action of gold shell-isolated nanoparticles against tumor cells depends on membrane interactions

Three-dimensional structure of an antigen-antibody complex

Investigation of simultaneously existed Raman scattering enhancement and inhibiting fluorescence using surface modified gold nanostars as SERS probes

Spectroscopic properties of molecules interacting with small dielectric particles

Increasing the enhancement factor in plasmon-enhanced fluorescence with shell-isolated nanoparticles

Study of the interaction between cardiolipin bilayers and methylene blue in polymer-based Layer-by-Layer and Langmuir films applied as membrane mimetic systems

Electromagnetic theory of enhanced Raman scattering by molecules adsorbed on rough surfaces

Surface-enhanced spectroscopy

Principles of surface-enhanced Raman spectroscopy and related plasmonic effects

Vibrational spectroscopy and surfaceenhanced vibrational spectroscopy of carbonaceous materials: from nanotubes to graphite, in: Opt. Pura y Apl

Co-deposition of gold nanoparticles and metalloporphyrin using the langmuir-blodgett (LB) technique for surfaceenhanced raman scattering (SERS)

Surface-enhanced Raman scattering of methylene blue adsorbed on cap-shaped silver nanoparticles

Rapid detection of trace methylene blue and malachite green in four fish tissues by ultra-sensitive surface-enhanced Raman spectroscopy coated with gold nanorods

A real-time surface enhanced Raman spectroscopy study of plasmonic photothermal cell death using targeted gold nanoparticles

Nondestructive detection of the freshness of fruits and vegetables using gold and silver nanoparticle mediated graphene enhanced Raman spectroscopy

Fluorescence and Fourier Transform surface-enhanced Raman scattering measurements of methylene blue adsorbed onto a sulfur-modified gold electrode

A novel rapid detection for SARS-CoV-2 spike 1 antigens using human angiotensin converting enzyme 2 (ACE2)

Rapid SARS-CoV-2 spike protein detection by carbon nanotube-based near-infrared nanosensors

Aspects of nano-enabling biosensing systems for intelligent healthcare; towards Covid-19 management

On a theory of the van der Waals adsorption of gases

Detection and quantitative analysis of carbendazim herbicide on Ag nanoparticles via surface-enhanced Raman scattering

Detection of thiabendazole fungicide/parasiticide by SERS: Quantitative analysis and adsorption mechanism

Surface geometry change in 2-naphthoic acid adsorbed on silver

Increasing the sensitivity of surface-enhanced Raman scattering detection for s-triazine pesticides by taking advantage of interactions with soil humic substances

Analytical Procedures and Methods Validation: Chemistry, Manufacturing, and Controls

Developing a J o u r n a l P r e -p r o o f paper-based antigen assay to differentiate between coronaviruses and SARS-CoV-2 spike variants

Rapid point-of-care Covid-19 diagnosis with a gold-nanoarchitecture-assisted laser-scribed graphene biosensor

There are no conflicts to declare.J o u r n a l P r e -p r o o f