Plasmonic nanoprobes for SERS biosensing and bioimaging

Recent Advances in Metallic Nanostructures-assisted Biosensors for Medical Diagnosis and Therapy

The field of nanotechnology has witnessed remarkable progress in recent years, particularly in its application to medical diagnosis and therapy. Metallic nanostructures-assisted biosensors have emerged as a powerful and versatile platform, offering unprecedented opportunities for sensitive, specific, and minimally invasive diagnostic techniques, as well as innovative therapeutic interventions. These biosensors exploit the molecular interactions occurring between biomolecules, such as antibodies, enzymes, aptamers, or nucleic acids, and metallic surfaces to induce observable alterations in multiple physical attributes, encompassing electrical, optical, colorimetric, and electrochemical signals. These interactions yield measurable data concerning the existence and concentration of particular biomolecules. The inherent characteristics of metal nanostructures, such as conductivity, plasmon resonance, and catalytic activity, serve to amplify both sensitivity and specificity in these biosensors. This review provides an in-depth exploration of the latest advancements in metallic nanostructures-assisted biosensors, highlighting their transformative impact on medical science and envisioning their potential in shaping the future of personalized healthcare.

Imaging of Antiferroelectric Dark Modes in an Inverted Plasmonic Lattice

Plasmonic lattice nanostructures are of technological interest because of their capacity to manipulate light below the diffraction limit. Here, we present a detailed study of dark and bright modes in the visible and near-infrared energy regime of an inverted plasmonic honeycomb lattice by a combination of Au+ focused ion beam lithography with nanometric resolution, optical and electron spectroscopy, and finite-difference time-domain simulations. The lattice consists of slits carved in a gold thin film, exhibiting hotspots and a set of bright and dark modes. We proposed that some of the dark modes detected by electron energy-loss spectroscopy are caused by antiferroelectric arrangements of the slit polarizations with two times the size of the hexagonal unit cell. The plasmonic resonances take place within the 0.5-2 eV energy range, indicating that they could be suitable for a synergistic coupling with excitons in two-dimensional transition metal dichalcogenides materials or for designing nanoscale sensing platforms based on near-field enhancement over a metallic surface.

Emerging trends in point-of-care biosensing strategies for molecular architectures and antibodies of SARS-CoV-2

COVID-19, a highly contagious viral infection caused by the occurrence of severe acute respiratory syndrome coronavirus (SARS-CoV-2), has turned out to be a viral pandemic then ravaged many countries worldwide. In the recent years, point-of-care (POC) biosensors combined with state-of-the-art bioreceptors, and transducing systems enabled the development of novel diagnostic tools for rapid and reliable detection of biomarkers associated with SARS-CoV-2. The present review thoroughly summarises and discusses various biosensing strategies developed for probing SARS-CoV-2 molecular architectures (viral genome, S Protein, M protein, E protein, N protein and non-structural proteins) and antibodies as a potential diagnostic tool for COVID-19. This review discusses the various structural components of SARS-CoV-2, their binding regions and the bioreceptors used for recognizing the structural components. The various types of clinical specimens investigated for rapid and POC detection of SARS-CoV-2 is also highlighted. The importance of nanotechnology and artificial intelligence (AI) approaches in improving the biosensor performance for real-time and reagent-free monitoring the biomarkers of SARS-CoV-2 is also summarized. This review also encompasses existing practical challenges and prospects for developing new POC biosensors for clinical monitoring of COVID-19.

Machine learning using convolutional neural networks for SERS analysis of biomarkers in medical diagnostics

Surface-enhanced Raman spectroscopy (SERS) has wide diagnostic applications because of narrow spectral features that allow multiplexed analysis. Machine learning (ML) has been used for non-dye-labeled SERS spectra but has not been applied to SERS dye-labeled materials with known spectral shapes. Here, we compare the performances of spectral decomposition, support vector regression, random forest regression, partial least squares regression, and convolutional neural network (CNN) for SERS "spectral unmixing" from a multiplexed mixture of 7 SERS-active "nanorattles" loaded with different dyes for mRNA biomarker detection. We showed that CNN most accurately determined relative contributions of each distinct dye-loaded nanorattle. CNN and comparative models were then used to analyze SERS spectra from a singleplexed, point-of-care assay detecting an mRNA biomarker for head and neck cancer in 20 samples. The CNN, trained on simulated multiplexed data, determined the correct dye contributions from the singleplex assay with RMSElabel = 6.42 × 10-2. These results demonstrate the potential of CNN-based ML to advance SERS-based diagnostics.

The onset of surface-enhanced Raman scattering for single-particle detection of submicroplastics

Microplastics represent an emerging environmental problem worldwide, raising ecological and food safety concerns. Compared to microplastics, there is growing evidence of an even higher abundance of submicro- and nanoplastics in the environment, but a reliable monitoring method for detecting these smaller-sized plastics is lacking. Herein we presented the application of surface-enhanced Raman scattering (SERS) for this purpose. Particles of polystyrene (PS; 600 nm) were used as the probe analyte. Gold nanourchins (AuNU; 50 nm), i.e. urchin-shaped nanoparticles with irregular spikes around the core, were used as the SERS-active substrate. The effectiveness of SERS on PS was evaluated at a single-particle level with different numbers of AuNU in order to determine the minimum conditions required for the onset of the SERS effect. Our findings suggest that SERS of a single particle of PS can be induced by as few as 1-5 particles of AuNU, and that the use of excitation wavelength at 785 nm is appropriate to meet the red-shifted surface plasmon resonance of AuNU upon aggregation. These specifications provide additional information for the development of SERS-based tools for detecting plastic particles < 1 µm in food and environmental samples.

Advances in Waveguide Bragg Grating Structures, Platforms, and Applications: An Up-to-Date Appraisal

A Bragg grating (BG) is a one-dimensional optical device that may reflect a specific wavelength of light while transmitting all others. It is created by the periodic fluctuation of the refractive index in the waveguide (WG). The reflectivity of a BG is specified by the index modulation profile. A Bragg grating is a flexible optical filter that has found broad use in several scientific and industrial domains due to its straightforward construction and distinctive filtering capacity. WG BGs are also widely utilized in sensing applications due to their easy integration and high sensitivity. Sensors that utilize optical signals for sensing have several benefits over conventional sensors that use electric signals to achieve detection, including being lighter, having a strong ability to resist electromagnetic interference, consuming less power, operating over a wider frequency range, performing consistently, operating at a high speed, and experiencing less loss and crosstalk. WG BGs are simple to include in chips and are compatible with complementary metal-oxide-semiconductor (CMOS) manufacturing processes. In this review, WG BG structures based on three major optical platforms including semiconductors, polymers, and plasmonics are discussed for filtering and sensing applications. Based on the desired application and available fabrication facilities, the optical platform is selected, which mainly regulates the device performance and footprint.

Super-resolution Imaging of Plasmonic Near-Fields: Overcoming Emitter Mislocalizations

Plasmonic nano-objects have shown great potential in enhancing applications like biological/chemical sensing, light harvesting and energy transfer, and optical/quantum computing. Therefore, an extensive effort has been vested in optimizing plasmonic systems and exploiting their field enhancement properties. Super-resolution imaging with quantum dots (QDs) is a promising method to probe plasmonic near-fields but is hindered by the distortion of the QD radiation pattern. Here, we investigate the interaction between QDs and "L-shaped" gold nanoantennas and demonstrate both theoretically and experimentally that this strong interaction can induce polarization-dependent modifications to the apparent QD emission intensity, polarization, and localization. Based on FDTD simulations and polarization-modulated single-molecule microscopy, we show that the displacement of the emitter's localization is due to the position-dependent interference between the emitter and the induced dipole, and can be up to 100 nm. Our results help pave a pathway for higher precision plasmonic near-field mapping and its underlying applications.

Diagnostic assay and technology advancement for detecting SARS-CoV-2 infections causing the COVID-19 pandemic

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-caused COVID-19 pandemic has transmitted to humans in practically all parts of the world, producing socio-economic turmoil. There is an urgent need for precise, fast, and affordable diagnostic testing to be widely available for detecting SARS-CoV-2 and its mutations in various phases of the disease. Early diagnosis with great precision has been achieved using real-time polymerase chain reaction (RT-PCR) and similar other molecular methods, but theseapproaches are costly and involve rigorous processes that are not easily obtainable. Conversely, immunoassays that detect a small number of antibodies have been employed for quick, low-cost tests, but their efficiency in diagnosing infected people has been restricted. The use of biosensors in the detection of SARS-CoV-2 is vital for the COVID-19 pandemic’s control. This review gives an overview of COVID-19 diagnostic approaches that are currently being developed as well as nanomaterial-based biosensor technologies, to aid future technological advancement and innovation. These approaches can be integrated into point-of-care (POC) devices to quickly identify a large number of infected patients and asymptomatic carriers. The ongoing research endeavors and developments in complementary technologies will play a significant role in curbing the spread of the COVID-19 pandemic and fill the knowledge gaps in current diagnostic accuracy and capacity.

Porous Au-Ag Nanoparticles from Galvanic Replacement Applied as Single-Particle SERS Probe for Quantitative Monitoring

Plasmonic nanostructures have raised the interest of biomedical applications of surface-enhanced Raman scattering (SERS). To improve the enhancement and produce sensitive SERS probes, porous Au-Ag alloy nanoparticles (NPs) are synthesized by dealloying Au-Ag alloy NP-precursors with Au or Ag core in aqueous colloidal environment through galvanic replacement reaction. The novel designed core-shell Au-Ag alloy NP-precursors facilitate controllable synthesis of porous nanostructure, and dealloying degree during the reaction has significant effect on structural and spectral properties of dealloyed porous NPs. Narrow-dispersed dealloyed NPs are obtained using NPs of Au/Ag ratio from 10/90 to 40/60 with Au and Ag core to produce solid core@porous shell and porous nanoshells, having rough surface, hollowness, and porosity around 30-60%. The clean nanostructure from colloidal synthesis exhibits a redshifted plasmon peak up to near-infrared region, and the large accessible surface induces highly localized surface plasmon resonance and generates robust SERS activity. Thus, the porous NPs produce intensely enhanced Raman signal up to 68-fold higher than 100 nm AuNP enhancement at single-particle level, and the estimated Raman enhancement around 7800, showing the potential for highly sensitive SERS probes. The single-particle SERS probes are effectively demonstrated in quantitative monitoring of anticancer drug Doxorubicin release.

An Inverted Honeycomb Plasmonic Lattice as an Efficient Refractive Index Sensor

We present an efficient refractive index sensor consisting of a heterostructure that contains an Au inverted honeycomb lattice as a main sensing element. Our design aims at maximizing the out-of-plane near-field distributions of the collective modes of the lattice mapping the sensor surroundings. These modes are further enhanced by a patterned SiO₂ layer with the same inverted honeycomb lattice, an SiO₂ spacer, and an Au mirror underneath the Au sensing layer that contribute to achieving a high performance. The optical response of the heterostructure was studied by numerical simulation. The results corresponding to one of the collective modes showed high sensitivity values ranging from 99 to 395 nm/RIU for relatively thin layers of test materials within 50 and 200 nm. In addition, the figure of merit of the sensor detecting slight changes of the refractive index of a water medium at a fixed wavelength was as high as 199 RIU^-1. As an experimental proof of concept, the heterostructure was manufactured by a simple method based on electron beam lithography and the measured optical response reproduces the simulations. This work paves the way for improving both the sensitivity of plasmonic sensors and the signal of some enhanced surface spectroscopies.

Optical Biosensors for Virus Detection: Prospects for SARS-CoV-2/COVID-19

The recent pandemic of the novel coronavirus disease 2019 (COVID-19) has caused huge worldwide disruption due to the lack of available testing locations and equipment. The use of optical techniques for viral detection has flourished in the past 15 years, providing more reliable, inexpensive, and accurate detection methods. In the current minireview, optical phenomena including fluorescence, surface plasmons, surface-enhanced Raman scattering (SERS), and colorimetry are discussed in the context of detecting virus pathogens. The sensitivity of a viral detection method can be dramatically improved by using materials that exhibit surface plasmons or SERS, but often this requires advanced instrumentation for detection. Although fluorescence and colorimetry lack high sensitivity, they show promise as point-of-care diagnostics because of their relatively less complicated instrumentation, ease of use, lower costs, and the fact that they do not require nucleic acid amplification. The advantages and disadvantages of each optical detection method are presented, and prospects for applying optical biosensors in COVID-19 detection are discussed.

Responsive Plasmonic Nanomaterials for Advanced Cancer Diagnostics

Plasmonic nanostructures, particularly of noble-metal Au and Ag, have attracted long-lasting research interests because of their intriguing physical and chemical properties. Under light excitation, their conduction electrons can form collective oscillation with the electromagnetic fields at particular wavelength, leading to localized surface plasmon resonance (LSPR). The remarkable characteristic of LSPR is the absorption and scattering of light at the resonant wavelength and greatly enhanced electric fields in localized areas. In response to the chemical and physical changes, these optical properties of plasmonic nanostructures will exhibit drastic color changes and highly sensitive peak shifts, which has been extensively used for biological imaging and disease treatments. In this mini review, we aim to briefly summarize recent progress of preparing responsive plasmonic nanostructures for biodiagnostics, with specific focus on cancer imaging and treatment. We start with typical synthetic approaches to various plasmonic nanostructures and elucidate practical strategies and working mechanism in tuning their LSPR properties. Current achievements in using responsive plasmonic nanostructures for advanced cancer diagnostics will be further discussed. Concise perspectives on existing challenges in developing plasmonic platforms for clinic diagnostics is also provided at the end of this review.

Ultrahigh-figure-of-merit refractive index sensor based on the Rayleigh anomaly resonance

We investigate an all-metal and simple-fabrication grating with an ultranarrow band absorption spectrum in the telecom window range. The influences of structure parameters on the absorption characteristics are investigated. For the best design, the absorption efficiency reaches 94% under normal incidence, with the full width at half-maximum of only 0.17 nm. We demonstrate that this ultranarrow band absorption is the result of the dominant excitation of the Rayleigh anomaly mode. The corresponding figure of merit is calculated to be 8530RIU^-1. The applied procedure has the potential to also be used in designing high-performance reflection-based sensors in other wavelength ranges.

Controllable Frequency Dependence of Resonance Energy Transfer Coupled with Localized Surface Plasmon Polaritons

We investigate the intrinsic characteristics of resonance energy transfer (RET) coupled with localized surface plasmon polaritons (LSPPs) from the perspective of macroscopic quantum electrodynamics. To quantify the effect of LSPPs, we propose a numerical scheme that allows us to accurately calculate the rate of RET between a donor-acceptor pair near a nanoparticle. Our study shows that LSPPs can be used to enhance the RET rate significantly and control its frequency dependence by modifying a core/shell structure, which indicates the possibility of RET rate optimization. Moreover, we systematically explore the angle (distance) dependence of the RET rate and analyze its origin. According to different frequency regimes, the angle dependence of RET is dominated by different mechanisms, such as LSPPs, surface plasmon polaritons (SPPs), and anti-resonance. For the proposed core/shell structure, the characteristic distance of RET coupled with LSPPs (approximately 0.05 emission wavelength) is shorter than that of RET coupled with SPPs (approximately 0.1 emission wavelength), which may provide promising applications in energy science.

Gold nanostars-diagnosis, bioimaging and biomedical applications

Gold Nanostars (GNS) have attracted tremendous attention toward themselves owing to their multi-branched structure and unique properties. These state of the art metallic nanoparticles possess intrinsic features like remarkable optical properties and exceptional physiochemical activities. These star-shaped gold nanoparticles can predominantly be utilized in biosensing, photothermal therapy, imaging, surface-enhanced Raman spectroscopy and target drug delivery applications due to their low toxicity and extraordinary optical features. In the current review, recent approaches in the matter of GNS in case of diagnosis, bioimaging and biomedical applications were summarized and reported. In this regard, first an overview about the structure and general properties of GNS were reported and thence detailed information regarding the diagnostic, bioimaging, photothermal therapy, and drug delivery applications of such novel nanomaterials were presented in detail. Summarized information clearly highlighting the superior capability of GNS as potential multi-functional materials for biomedical applications.

On-Demand Electromagnetic Hotspot Generation in Surface-Enhanced Raman Scattering Substrates via "Add-On" Plasmonic Patch

Electromagnetic hotspots at the interstices of plasmonic assemblies are recognized to be the most potent sites for surface-enhanced Raman scattering (SERS). We demonstrate a novel "add-on" electromagnetic hotspot formation technique, which significantly improves the sensitivity of conventional SERS substrates composed of individual plasmonic nanostructures. The novel approach demonstrated here involves the transfer of "plasmonic patch", a transparent, flexible, and conformal elastomeric film adsorbed with plasmonic nanostructures, onto a conventional SERS substrate. The addition of the plasmonic patch onto a conventional SERS substrate following the analyte capture results in the formation of electromagnetic hotspots and hence a large SERS enhancement. The application of the plasmonic patch improves the sensitivity and limit of detection of conventional SERS substrates by up to ∼100-fold. The transfer of the plasmonic patch also effectively transforms the SERS-inactive gold mirror to a highly SERS-active "particle-on-mirror" system. Furthermore, we demonstrate that the "add-on" technique can be effectively utilized for the vapor-phase detection of explosives such as trinitrotoluene (TNT) using peptide recognition elements. We believe that the on-demand hotspot formation approach presented here represents a highly versatile and ubiquitously applicable technology readily expandable to any existing SERS substrate without employing complicated modification.

DNA-Functionalized Plasmonic Nanomaterials for Optical Biosensing

Plasmonic nanomaterials, especially Au and Ag nanomaterials, have shown attractive physicochemical properties, such as easy functionalization and tunable optical bands. The development of this active subfield paves the way to the fascinating biosensing platforms. In recent years, plasmonic nanomaterials-based sensors have been extensively investigated because they are useful for genetic diseases, biological processes, devices, and cell imaging. In this account, a brief introduction of the development of optical biosensors based on DNA-functionalized plasmonic nanomaterials is presented. Then the common strategies for the application of the optical sensors are summarized, including colorimetry, fluorescence, localized surface plasmon resonance, and surface-enhanced resonance scattering detection. The focus is on the fundamental aspect of detection methods, and then a few examples of each method are highlighted. Finally, the opportunities and challenges for the plasmonic nanomaterials-based biosensing are discussed with the development of modern technologies.

Fabrication of gold-silver core-shell nanoparticles for performing as ultrabright SERS-nanotags inside human ovarian cancer cells

This paper presents the fabrication and characterization of new gold-silver core-shell nanoparticles labeled with para-mercaptobenzoic acid (4MBA) molecules and demonstrates their use as surface-enhanced Raman spectroscopy (SERS)-nanotags with ultra-bright traceability inside cells and ability to convey spectrally-coded information about the intracellular pH by means of SERS. Unlike previous reported studies, our fabrication procedure includes in the first step the synthesis of chitosan-coated gold nanoparticles as a seed material with subsequent growing of a silver shell. The bimetallic core-shell structure is revealed by transmission electron microscopy, high-angle annular dark field scanning transmission electron microscopy, energy-dispersive x-ray elemental mapping and the presence of two interacting localized surface plasmon resonance modes in UV-vis extinction spectrum. The high SERS activity and sensitivity of as fabricated 4MBA-chit-Au-AgNPs nano-constructs to different pH in solution is investigated under 532 and 633 nm laser lines excitation. Next, in view of future studies in cancer diagnosis, the in vitro antiproliferative effects of SERS-nanotags against human ovarian adenocarcinoma cells (NIH:OVCAR-3) are evaluated. The capacity to operate as bright SERS nanotags with precise localization at a single cell level as well as intracellular pH indicators is clearly demonstrated by performing cell imaging under scanning confocal Raman microscopy.

Bifunctional Fluorescent/Raman Nanoprobe for the Early Detection of Amyloid

One of the pathological hallmarks of Alzheimer's disease (AD) is the abnormal aggregation of amyloid beta (Aβ) peptides. Therefore the detection of Aβ peptides and imaging of amyloid plaques are considered as promising diagnostic methods for AD. Here we report a bifunctional nanoprobe prepared by conjugating gold nanoparticles (AuNPs) with Rose Bengal (RB) dye. RB is chosen due to its unique Raman fingerprints and affinity with Aβ peptides. After the conjugation, Raman signals of RB were significantly enhanced due to the surface-enhanced Raman scattering (SERS) effect. Upon binding with Aβ42 peptides, a spectrum change was detected, and the magnitude of the spectrum changes can be correlated with the concentration of target peptides. The peptide/probe interaction also induced a remarkable enhancement in the probes' fluorescence emission. This fluorescence enhancement was further utilized to image amyloid plaques in the brain slices from transgenic mice. In this study, the RB-AuNPs were used for both SERS-based detection of Aβ42 peptides and fluorescence-based imaging of amyloid plaques. Compared to monofunctional probes, the multifunctional probe is capable to provide more comprehensive pathophysiological information, and therefore, the implementation of such multifunctional amyloid probes is expected to help the investigation of amyloid aggregation and the early diagnosis of AD.

Point of care testing for infectious diseases

Infectious diseases are caused by pathogenic microorganisms and can be transmitted between individuals and populations thus threatening the general public health and potentially the economy. Efficient diagnostic tools are needed to provide accurate and timely guidance for case identification, transmission disruption and appropriate treatment administration. Point of care (POC) tests provide actionable results near the patient and thereby serve as a personal "radar". In this review, we review clinical needs for POC testing for several major pathogens, including malaria parasites, human immunodeficiency virus (HIV), human papillomavirus (HPV), dengue, Ebola and Zika viruses and Mycobacterium tuberculosis (TB). We compare different molecular approaches, including pathogen nucleic acid and protein, circulating microRNA and antibodies, used in the POC tests. Finally, we review recent advances in novel POC technologies focusing on microfluidic and plasmonic-based approaches.

Plasmonics for Biosensing

Techniques based on plasmonic resonance can provide label-free, signal enhanced, and real-time sensing means for bioparticles and bioprocesses at the molecular level. With the development in nanofabrication and material science, plasmonics based on synthesized nanoparticles and manufactured nano-patterns in thin films have been prosperously explored. In this short review, resonance modes, materials, and hybrid functions by simultaneously using electrical conductivity for plasmonic biosensing techniques are exclusively reviewed for designs containing nanovoids in thin films. This type of plasmonic biosensors provide prominent potential to achieve integrated lab-on-a-chip which is capable of transporting and detecting minute of multiple bio-analytes with extremely high sensitivity, selectivity, multi-channel and dynamic monitoring for the next generation of point-of-care devices.

A repertoire of biomedical applications of noble metal nanoparticles

The emerging properties of noble metal nanoparticles are attracting huge interest from the translational scientific community. In this feature article, we highlight recent advances in the adaptation of noble metal nanomaterials and their biomedical applications in therapeutics, diagnostics and sensing.

Label-free detection of nasopharyngeal and liver cancer using surface-enhanced Raman spectroscopy and partial lease squares combined with support vector machine

In this paper, we investigated the feasibility of using surface enhanced Raman spectroscopy (SERS) and multivariate analysis method to discriminate liver cancer and nasopharyngeal cancer from healthy volunteers. SERS measurements were performed on serum protein samples from 104 liver cancer patients, 100 nasopharyngeal cancer patients, and 95 healthy volunteers. Two dimensionality reduction methods, principal component analysis (PCA) and partial least square (PLS) were compared, and the results indicated that the performance of PLS is superior to that of PCA. When the number of components was compressed to 3 by PLS, support vector machine (SVM) with a Gaussian radial basis function (RBF) was employed to classify various cancers simultaneously. Based on the PLS-SVM algorithm, high diagnostic accuracies of 95.09% and 90.67% were achieved from the training set and the unknown testing set, respectively. The results of this exploratory work demonstrate that serum protein SERS technology combined with PLS-SVM diagnostic algorithm has great potential for the noninvasive screening of cancer.

The Role of Rayleigh-Wood Anomalies and Surface Plasmons in Optical Enhancement for Nano-Gratings

We propose and report on the design of a 1-D metallo-dielectric nano-grating on a GaAs substrate. We numerically study the impact of grating period, slit and wire widths, and irradiating angle of incidence on the optical response. The optimal wire width, w = 160 nm, was chosen based on previous results from investigations into the influence of wire width and nano-slit dimensions on optical and electrical enhancements in metal-semiconductor-metal photodetectors. In this present project, resonant absorption and reflection modes were observed while varying the wire and nano-slit widths to study the unique optical modes generated by Rayleigh-Wood anomalies and surface plasmon polaritons. We observed sharp and diffuse changes in optical response to these anomalies, which may potentially be useful in applications such as photo-sensing and photodetectors. Additionally, we found that varying the slit width produced sharper, more intense anomalies in the optical spectrum than varying the wire width.

Functionalized Gold Nanoparticles as Biosensors for Monitoring Cellular Uptake and Localization in Normal and Tumor Prostatic Cells

In the present contribution the fabrication and characterization of functionalized gold nanospheres of uniform shape and controlled size is reported. These nano-objects are intended to be used as Surface Enhanced Raman Spectroscopy (SERS) sensors for in-vitro cellular uptake and localization. Thiophenol was used as molecular reporter and was bound to the Au surface by a chemisorption process in aqueous solution. The obtained colloidal solution was highly stable and no aggregation of the single nanospheres into larger clusters was observed. The nanoparticles were incubated in human prostatic cells with the aim of developing a robust, SERS-based method to differentiate normal and tumor cell lines. SERS imaging experiments showed that tumor cells uptake considerably larger amounts of nanoparticles in comparison to normal cells (up to 950% more); significant differences were also observed in the uptake kinetics. This largely different behaviour might be exploited in diagnostic and therapeutic applications.

Tunable Au-Ag nanobowl arrays for size-selective plasmonic biosensing

Selectivity is often a major obstacle for localized surface plasmon resonance-based biosensing in complex biological solutions. An additional degree of selectivity can be achieved through the incorporation of shape complementarity on the nanoparticle surface. Here, we report the versatile fabrication of substrate-bound Au-Ag nanobowl arrays through the galvanic ion replacement of silver nanodisk arrays. Both localized surface plasmon resonance (LSPR) and surface enhanced Raman spectroscopy (SERS) were carried out to detect the binding of analytes of varying size to the nanobowl arrays. Large increases in the LSPR and SERS response were measured for analytes that were small enough to enter the nanobowls, compared to those too large to come into contact with the interior of the nanobowls. This size-selective sensing should prove useful in both size determination and differentiation of large analytes in biological solutions, such as viruses, fungi, and bacterial cells.

Silver nanowires as infrared-active materials for surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) is increasing in significance as a bioanalytical tool. Novel nanostructured metal substrates are required to improve performances and versatility of SERS spectroscopy. In particular, as biological tissues are relatively transparent in the infrared wavelength range, SERS-active materials suitable for infrared laser excitation are needed. Nanowires appear interesting in this respect as they show a very broad localized surface plasmon resonance band, ranging from near UV to near infrared wavelengths. The SERS activity of silver nanowires has been tested at three wavelengths and a fair enhancement at 1064 and 514 nm has been observed, whereas a very weak enhancement was present when exciting close to the nanowire extinction maximum. These experimentally measured optical properties have been contrasted with finite element method simulations. Furthermore, laser-induced optoacoustic spectroscopy measurements have shown that the extinction at 1064 nm is completely due to scattering. This result has an important implication that no heating occurs when silver nanowires are utilized as SERS-active substrates, thereby preventing possible thermal damage.

The plasmonic properties of gold nanoparticle clusters formed via applying an AC electric field

An external electric AC field with a field strength ranging from 10 V mm-1 to 30 V mm-1 and a frequency ranging from 0.1 kHz to 1 MHz was applied to suspensions of gold nanoparticles (Au NPs) to control their plasmonic properties. Apparent differences in the UV-vis spectra of the Au NPs were observed between the spectra with and without the field application. The characteristic red color of the Au NP suspension darkened; this suggested that the application of the AC field caused the aggregation of the Au NPs. In addition, the sizes of the Au NP clusters in suspension formed by the AC field application depended on the frequency of the applied field. The surface-enhanced Raman scattering (SERS) effects of Au NP clusters were examined by comparing the difference in Raman intensities obtained at 30 V mm-1 and in a frequency range of 0.1 kHz to 1 MHz. The application of a low-frequency field at 0.1 kHz caused a rapid aggregation of the Au NPs, resulting in low Raman intensities of the probe molecules. Conversely, high-frequency applications between 1 kHz and 1 MHz successfully enhanced the Raman intensities of the molecules in suspension. The strong correlation of the optical/sensing properties with the Au NP clustering states reveals that the application of an AC electric field is a powerful tool for control over the plasmonic properties.

Fabrication, characterization, and high temperature surface enhanced Raman spectroscopic performance of SiO2 coated silver particles

We present a systematic study on the fabrication, characterization and high temperature surface enhanced Raman spectroscopy (SERS) performance of SiO₂ coated silver nanoparticles (Ag@SiO₂) on a flat substrate, aiming to obtain a thermally robust SERS substrate for monitoring high temperature reactions. We confirm that a 10-15 nm SiO₂ coating provides a structure stability up to 900 °C without significantly sacrificing the enhancement factor, while the uncoated particle cannot retain the SERS effect above 500 °C. The finite difference time domain (FDTD) simulation results supported that the SiO₂ coating almost has no influence on the distribution of the electric field but only physically trapped the most enhanced spot inside the coating layer. On this thermally robust substrate, we confirmed that the SERS of horizontally aligned single walled carbon nanotubes is stable at elevated temperatures, and demonstrate an in situ Raman monitoring of the atmosphere of the annealing process of nanodiamonds, in which the interconverting process of C-C bonds is unambiguously observed. We claim that this is a first experimental proof that the high temperature SERS effect can be preserved and applied in a chemical reaction at temperature above 500 °C. This versatile substrate also enables novel opportunities for observing growth, etching, and structure transformation of many 0D and 2D nano-materials.

Gold nanorods decorated with a cancer drug for multimodal imaging and therapy

Cancer, a condition with uncontrolled cell division, is the second leading cause of death worldwide. The currently available techniques for the imaging and treatment of cancer have their own limitations and hence a combination of more than one modality is expected to increase the efficacy of both diagnosis and treatment. In the present study, we have developed a multimodal imaging and therapeutic system by incorporating a chemotherapeutic drug, mitoxantrone (MTX) onto PEG coated gold nanorods (GNR). Strong absorption in the near-infrared (NIR) and visible regions qualifies GNR as an efficient photothermal (PTT) agent upon irradiation with either a NIR or visible laser. Additionally, the enhanced electric field of GNR makes it a suitable substrate for surface enhanced Raman scattering (SERS). Modification of GNR with amino PEG offers biocompatibility without affecting its optical property. In order to achieve tumor specificity, GNR-PEG was conjugated with tumor specific marker that can target cancer cells, leaving the normal cells unaffected. The incorporation of fluorescent chemotherapeutic drug mitoxantrone onto GNR-PEG facilitates chemotherapy as well as fluorescence imaging. The therapeutic efficacy of the developed GNR based system is tracked using fluorescence imaging and Raman imaging. The careful design of the system also facilitates the controlled release of the drug by photothermal triggering. Likewise, the imaging modality could be chosen as either Raman or fluorescence to monitor drug release in accordance with irradiation. The physico-chemical properties, and drug release profiles under different physiological conditions have been well studied. Finally, the developed system was tested for its therapeutic efficacy using cancer cells, in vitro. The receptor mediated cell uptake was more effective in folate receptor over-expressing cancer cells than in the normal and low-expressing cells. Accordingly the percentage of cell death was higher in folate receptor over-expressing cancer cells, which was further enhanced due to the effect of the dual therapeutic approach. The cell uptake and treatment efficacy was monitored using fluorescence microscopy and SERS. In conclusion, the developed GNR-PEG-MTX system is found to be an efficient multimodal therapeutic agent against cancer which could be tracked using two different techniques.

Shining Gold Nanostars: From Cancer Diagnostics to Photothermal Treatment and Immunotherapy

Cancer has been a significant threat to human health with more than eight million deaths each year in the world. There is an urgent need to develop novel methods to improve cancer management. Biocompatible gold nanostars (GNS) with tip-enhanced electromagnetic and optical properties have been developed and applied for multifunctional cancer diagnostics and therapy (theranostics). The GNS platform can be used for multiple sensing, imaging and treatment modalities, such as surface-enhanced Raman scattering, two-photon photoluminescence, magnetic resonance imaging and computed tomography as well as photothermal therapy and immunotherapy. GNS-mediated photothermal therapy combined with checkpoint immunotherapy has been found to reverse tumor-mediated immunosuppression, leading to the treatment of not only primary tumors but also cancer metastasis as well as inducing effective long-lasting immunity, i.e. an anticancer 'vaccine' effect.

Application of nanoparticles in cancer detection by Raman scattering based techniques

In vitro detection technique Raman spectroscopy (Rs), in one number times another Rs  based expert ways of art and so on, are useful instruments for cancer  discovery. top gave greater value to Raman spectroscopy  sers is a relatively new careful way for in vitro  and in vivo discovery that takes away bad points of simple Raman spectroscopy (Rs). Raman spectroscopy (RS) and in particular, multiple RS-based techniques are useful for cancer detection. Surface enhanced Raman spectroscopy (SERS) is a relatively new method for both in vitro and in vivo detection, which eliminates the drawbacks of simple RS. Using nanoparticles has elevated the sensitivity and specificity of SERS. SERS has the potential to increase sensitivity, specificity and spatial resolution in cancer detection, especially in cooperation with other diagnostic imaging tools such as magnetic resonance imaging (MRI) and PET-scan polyethylene terephthalate. Developing a hand held instrument for detecting cancer or other illnesses may also be feasible by using SERS. Frequently, novel nanoparticles are used in SERS. With a focus on nanoparticle utilization, we review the benefits of RS in cancer detection and related biomarkers. With a focus on nanoparticles utilizations, the benefits of RS in cancer detection and related biomarkers were reviewed. In addition, Raman applications to detect some of prevalent were discussed. Also more investigated cancers such as breast and colorectal cancer, multiple nanostructures and their possible special biomarkers, especially as SERS nano-tag have been reviewed. The main purpose of this article is introducing of most popular nanotechnological approaches in cancer detection by using Raman techniques. Moreover, have been caught up on detection and reviewed some of the most prevalent and also more investigated cancers such as breast, colorectal cancer, multiple intriguing nanostructures, especially as SERS nano-tag, special cancer biomarkers and related approaches. The main purpose of this article is to introduce the most popular nanotechnological approaches in cancer detection by using Raman techniques.

Vertical stacking of three-dimensional nanostructures via an aerosol lithography for advanced optical applications

In this report, we introduce a method utilizing ion-assisted aerosol lithography to stack 3D nanostructures vertically. The stacked 3D nanostructures exhibit extraordinary optical properties: the double layer 3D nanostructures show more than 5-fold increased surface enhanced Raman scattering intensities compared to their single layer counterpart. This unusual enhancement of Raman intensity implies the existence of additional vertical hotspots formed by interlayer cavity effects between the lower and upper nanostructures. Allowing for full three-dimensional control in nanofabrication, this work provides a reliable way to create complex-shaped advanced optical nanostructures with non-intuitive bulk optical properties.

Leukemia cells detection based on electroporation assisted surface-enhanced Raman scattering

In this study, an electroporation-based surface-enhanced Raman scattering (SERS) technique was employed to differentiate the human myeloid leukemia cells from the normal human bone marrow mononuclear cells with the aim to develop a fast and label-free method for leukemia cell screening. The Ag nanoparticles were delivered into living cells by electroporation, and then high quality SERS spectra were successfully obtained from 60 acute promyelocytic leukemia cells (HL60 cell line), 60 chronic myelogenous leukemia cells (K562 cell line) and 60 normal human bone marrow mononuclear cells (BMC). Principal component analysis (PCA) combined with linear discriminant analysis (LDA) differentiated the leukemia cell SERS spectra (HL60 plus K562) from normal cell SERS spectra (BMC) with high sensitivity (98.3%) and specificity (98.3%). Furthermore, partial least squares (PLS) approach was employed to develop a diagnostic model. The model successfully predicted the unidentified subjects with a diagnostic accuracy of 96.7%. This exploratory work demonstrates that the electroporation-based SERS technique combined with PCA-LDA and PLS diagnostic algorithms possesses great promise for cancer cell screening.

Thin silica shell coated Ag assembled nanostructures for expanding generality of SERS analytes

Surface-enhanced Raman scattering (SERS) provides a unique non-destructive spectroscopic fingerprint for chemical detection. However, intrinsic differences in affinity of analyte molecules to metal surface hinder SERS as a universal quantitative detection tool for various analyte molecules simultaneously. This must be overcome while keeping close proximity of analyte molecules to the metal surface. Moreover, assembled metal nanoparticles (NPs) structures might be beneficial for sensitive and reliable detection of chemicals than single NP structures. For this purpose, here we introduce thin silica-coated and assembled Ag NPs (SiO2@Ag@SiO2 NPs) for simultaneous and quantitative detection of chemicals that have different intrinsic affinities to silver metal. These SiO2@Ag@SiO2 NPs could detect each SERS peak of aniline or 4-aminothiophenol (4-ATP) from the mixture with limits of detection (LOD) of 93 ppm and 54 ppb, respectively. E-field distribution based on interparticle distance was simulated using discrete dipole approximation (DDA) calculation to gain insight into enhanced scattering of these thin silica coated Ag NP assemblies. These NPs were successfully applied to detect aniline in river water and tap water. Results suggest that SiO2@Ag@SiO2 NP-based SERS detection systems can be used as a simple and universal detection tool for environment pollutants and food safety.

Structure-dependent SERS activity of plasmonic nanorattles with built-in electromagnetic hotspots

We investigate the surface enhanced Raman scattering (SERS) activity of plasmonic nanorattles obtained through different degrees of galvanic replacement of Au@Ag nanocubes.

Measurement of pH-dependent surface-enhanced hyper-Raman scattering at desired positions on yeast cells via optical trapping

At desired positions on yeast, pH-dependent surface-enhanced hyper-Raman scattering (SEHRS) spectra were recorded by focusing a near-infrared laser beam while silver nanoparticles (AgNPs) with 4-mercaptobenzoic acid (p-MBA) were simultaneously optically trapped.

Gold Nanoparticles for Diagnostics: Advances towards Points of Care

The remarkable physicochemical properties of gold nanoparticles (AuNPs) have prompted developments in the exploration of biomolecular interactions with AuNP-containing systems, in particular for biomedical applications in diagnostics. These systems show great promise in improving sensitivity, ease of operation and portability. Despite this endeavor, most platforms have yet to reach maturity and make their way into clinics or points of care (POC). Here, we present an overview of emerging and available molecular diagnostics using AuNPs for biomedical sensing that are currently being translated to the clinical setting.

Folate-based single cell screening using surface enhanced Raman microimaging

Recent progress in nanotechnology and its application to biomedical settings have generated great advantages in dealing with early cancer diagnosis. The identification of the specific properties of cancer cells, such as the expression of particular plasma membrane molecular receptors, has become crucial in revealing the presence and in assessing the stage of development of the disease. Here we report a single cell screening approach based on Surface Enhanced Raman Scattering (SERS) microimaging. We fabricated a SERS-labelled nanovector based on the biofunctionalization of gold nanoparticles with folic acid. After treating the cells with the nanovector, we were able to distinguish three different cell populations from different cell lines (cancer HeLa and PC-3, and normal HaCaT lines), suitably chosen for their different expressions of folate binding proteins. The nanovector, indeed, binds much more efficiently on cancer cell lines than on normal ones, resulting in a higher SERS signal measured on cancer cells. These results pave the way for applications in single cell diagnostics and, potentially, in theranostics.

Plasmonic Circuit Theory for Multiresonant Light Funneling to a Single Spatial Hot Spot

We present a theoretical framework, based on plasmonic circuit models, for generating a multiresonant field intensity enhancement spectrum at a single "hot spot" in a plasmonic device. We introduce a circuit model, consisting of an array of coupled LC resonators, that directs current asymmetrically in the array, and we show that this circuit can funnel energy efficiently from each resonance to a single element. We implement the circuit model in a plasmonic nanostructure consisting of a series of metal bars of differing length, with nearest neighbor metal bars strongly coupled electromagnetically through air gaps. The resulting nanostructure resonantly traps different wavelengths of incident light in separate gap regions, yet it funnels the energy of different resonances to a common location, which is consistent with our circuit model. Our work is important for a number of applications of plasmonic nanoantennas in spectroscopy, such as in single-molecule fluorescence spectroscopy or Raman spectroscopy.