Infrared spectroscopy of live cells from a flowing solution using electrically-biased plasmonic metasurfaces

Deformation induced evolution of plasmonic responses in polymer grafted nanoparticle thin films

Multi-functional nanoparticle thin films are being used in various applications ranging from biosensing to photo-voltaics. In this study, we integrate two different numerical approaches to understand the interplay between the mechanical deformation and optical response of polymer grafted plasmonic nanoparticle (PGPN) arrays. Using numerical simulations we examine the deformation of thin films formed by end-functionalised polymer grafted nanoparticles subject to uniaxial elongation. The induced deformation causes the particles in the thin film network to rearrange their positions by two different mechanisms viz. sliding and packing. In sliding, the particles move in the direction of induced deformation. On the other hand, in packing, the particles move in a direction normal to that of the induced deformation. By employing a Green's tensor formulation in polarizable backgrounds for evaluating the optical response of the nanoparticle network, we calculate the evolution of the plasmonic response of the structure as a function of strain. The results indicate that the evolution of plasmonic response closely follows the deformation. In particular, we show that the onset of relative electric field enhancement of the optical response occurs when there is significant rearrangement of the constituent PGPNs in the array. Furthermore, we show that depending on the local packing/sliding and the polarization of the incident light there can be both enhancement and suppression of the SERS response.

Inverted Transflection Spectroscopy of Live Cells Using Metallic Grating on Elevated Nanopillars

Water absorption of mid-infrared (MIR) radiation severely limits the options for vibrational spectroscopy of the analytes-including live biological cells-that must be probed in aqueous environments. While internal reflection elements, such as attenuated total reflection prisms and metasurfaces, partially overcome this limitation, such devices have their own limitations: ATR prisms are difficult to integrate with multiwell cell culture workflows, while metasurfaces suffer from a limited spectral range and small penetration depth into analytes. In this work, we introduce an alternative live cell biosensing platform based on metallic nanogratings fabricated on top of elevated dielectric pillars. For the MIR wavelengths that are significantly longer than the grating period, reflection-based spectroscopy enables broadband sensing of the analytes inside the trenches separating the dielectric pillars. Because the depth of the analyte twice-traversed by the MIR light excludes the highly absorbing thick water layer above the grating, we refer to the technique as inverted transflection spectroscopy (ITS). The analytic power of ITS is established by measuring a wide range of protein concentrations in solution, with the limit of detection in the single-digit mg mL-1. The ability of ITS to interrogate live cells that naturally wrap themselves around the grating is used to characterize their adhesion kinetic.

Inverted transflection spectroscopy of live cells using metallic grating on elevated nanopillars

Water absorption of mid-infrared (MIR) radiation severely limits the options for vibrational spectroscopy of the analytes - including live biological cells - that must be probed in aqueous environments. While internal reflection elements, such as attenuated total reflection prisms and metasurfaces, partially overcome this limitation, such devices have their own limitations: high cost, incompatibility with standard cell culture workflows, limited spectral range, and small penetration depth into the analyte. In this work, we introduce an alternative live cell biosensing platform based on metallic nanogratings fabricated atop elevated dielectric pillars. For the MIR wavelengths that are significantly longer than the grating period, reflection-based spectroscopy enables broadband sensing of the analytes inside the trenches separating the dielectric pillars. Because the depth of the analyte twice-traversed by the MIR light excludes the highly absorbing thick water layer above the grating, we refer to the technique as Inverted Transflection Spectroscopy (ITS). We demonstrate the analytic power of ITS by measuring protein concentrations in solution. The ability of ITS to interrogate live cells that naturally wrap themselves around the grating is also exploited to characterize their adhesion kinetics.

Metasurface-Enhanced Infrared Spectroscopy: An Abundance of Materials and Functionalities

Infrared spectroscopy provides unique information on the composition and dynamics of biochemical systems by resolving the characteristic absorption fingerprints of their constituent molecules. Based on this inherent chemical specificity and the capability for label-free, noninvasive, and real-time detection, infrared spectroscopy approaches have unlocked a plethora of breakthrough applications for fields ranging from environmental monitoring and defense to chemical analysis and medical diagnostics. Nanophotonics has played a crucial role for pushing the sensitivity limits of traditional far-field spectroscopy by using resonant nanostructures to focus the incident light into nanoscale hot-spots of the electromagnetic field, greatly enhancing light-matter interaction. Metasurfaces composed of regular arrangements of such resonators further increase the design space for tailoring this nanoscale light control both spectrally and spatially, which has established them as an invaluable toolkit for surface-enhanced spectroscopy. Starting from the fundamental concepts of metasurface-enhanced infrared spectroscopy, a broad palette of resonator geometries, materials, and arrangements for realizing highly sensitive metadevices is showcased, with a special focus on emerging systems such as phononic and 2D van der Waals materials, and integration with waveguides for lab-on-a-chip devices. Furthermore, advanced sensor functionalities of metasurface-based infrared spectroscopy, including multiresonance, tunability, dielectrophoresis, live cell sensing, and machine-learning-aided analysis are highlighted.

Metasurface-enhanced infrared spectroscopy in multiwell format for real-time assaying of live cells

Fourier transform infrared (FTIR) spectroscopy is a popular technique for the analysis of biological samples, yet its application in characterizing live cells is limited due to the strong attenuation of mid-IR light in water. Special thin flow cells and attenuated total reflection (ATR) FTIR spectroscopy have been used to mitigate this problem, but these techniques are difficult to integrate into a standard cell culture workflow. In this work, we demonstrate that the use of a plasmonic metasurface fabricated on planar substrates and the probing of cellular IR spectra through metasurface-enhanced infrared spectroscopy (MEIRS) can be an effective technique to characterize the IR spectra of live cells in a high-throughput manner. Cells are cultured on metasurfaces integrated with multiwell cell culture chambers and are probed from the bottom using an inverted FTIR micro-spectrometer. To demonstrate the use of MEIRS as a cellular assay, cellular adhesion on metasurfaces with different surface coatings and cellular response to the activation of the protease-activated receptor (PAR) signaling pathway were characterized through the changes in cellular IR spectra.

Non-invasive monitoring of biochemicals in hydrogel-assisted microfluidic chips

Microfluidic chips are prevailingly utilized in biochemical monitoring and clinical diagnostics due to their capability of manipulating minuscule amounts of liquids in a highly integrated manner. Fabrication of microchannels on chips is commonly based on glass or polydimethylsiloxane, and sensing of the fluids and biochemicals within them relies on invasive embedded sensing accessories in the channels. In this study, we propose a hydrogel-assisted microfluidic chip for non-invasive monitoring of chemicals in microfluidics. A nanoporous hydrogel acts as a perfect sealing film on top of a microchannel to encapsulate liquid, and allows for the delivery of target biochemicals to its surface, leaving an open window for non-invasive analysis. This functionally "open" microchannel can be integrated with various electrical, electrochemical, and optical methods to realize accurate detection of biochemicals, suggesting the potential of hydrogel microfluidic chips for non-invasive clinical diagnostics and smart healthcare.

Towards multi-molecular surface-enhanced infrared absorption using metal plasmonics

Surface-enhanced infrared absorption (SEIRA) leads to a largely improved detection of polar molecules, compared to standard infrared absorption. The enhancement principle is based on localized surface plasmon resonances of the substrate, which match the frequency of molecular vibrations in the analyte of interest. Therefore, in practical terms, the SEIRA sensor needs to be tailored to each specific analyte. We review SEIRA sensors based on metal plasmonics for the detection of biomolecules such as DNA, proteins, and lipids. We further focus this review on chemical SEIRA sensors, with potential applications in quality control, as well as on the improvement in sensor geometry that led to the development of multiresonant SEIRA substrates as sensors for multiple analytes. Finally, we give an introduction into the integration of SEIRA sensors with surface-enhanced Raman scattering (SERS).

Probing the Drug Dynamics of Chemotherapeutics Using Metasurface-Enhanced Infrared Reflection Spectroscopy of Live Cells

Infrared spectroscopy has drawn considerable interest in biological applications, but the measurement of live cells is impeded by the attenuation of infrared light in water. Metasurface-enhanced infrared reflection spectroscopy (MEIRS) had been shown to mitigate the problem, enhance the cellular infrared signal through surface-enhanced infrared absorption, and encode the cellular vibrational signatures in the reflectance spectrum at the same time. In this study, we used MEIRS to study the dynamic response of live cancer cells to a newly developed chemotherapeutic metal complex with distinct modes of action (MoAs): tricarbonyl rhenium isonitrile polypyridyl (TRIP). MEIRS measurements demonstrated that administering TRIP resulted in long-term (several hours) reduction in protein, lipid, and overall refractive index signals, and in short-term (tens of minutes) increase in these signals, consistent with the induction of endoplasmic reticulum stress. The unique tricarbonyl IR signature of TRIP in the bioorthogonal spectral window was monitored in real time, and was used as an infrared tag to detect the precise drug delivery time that was shown to be closely correlated with the onset of the phenotypic response. These results demonstrate that MEIRS is an effective label-free real-time cellular assay capable of detecting and interpreting the early phenotypic responses of cells to IR-tagged chemotherapeutics.

Liquid Biopsy and Dielectrophoretic Analysis—Complementary Methods in Skin Cancer Monitoring

The incidence and prevalence of skin cancers is currently increasing worldwide, with early detection, adequate treatment, and prevention of recurrences being topics of great interest for researchers nowadays. Although tumor biopsy remains the gold standard of diagnosis, this technique cannot be performed in a significant proportion of cases, so that the use of alternative methods with high sensitivity and specificity is becoming increasingly desirable. In this context, liquid biopsy appears to be a feasible solution for the study of cellular and molecular markers relevant to different types of skin cancers. Circulating tumor cells are just one of the components of interest obtained from performing liquid biopsy, and their study by complementary methods, such as dielectrophoresis, could bring additional benefits in terms of characterizing skin tumors and subsequently applying personalized therapy. One purpose of this review is to demonstrate the utility of liquid biopsy primarily in monitoring the most common types of skin tumors: basal cell carcinoma, squamous cell carcinoma, and malign melanoma. In addition, the originality of the article is based on the detailed presentation of the dielectrophoretic analysis method of the most important elements obtained from liquid biopsy, with direct impact on the clinical and therapeutic approach of skin tumors.

Monitoring the effects of chemical stimuli on live cells with metasurface-enhanced infrared reflection spectroscopy

Infrared spectroscopy has found wide applications in the analysis of biological materials. A more recent development is the use of engineered nanostructures - plasmonic metasurfaces - as substrates for metasurface-enhanced infrared reflection spectroscopy (MEIRS). Here, we demonstrate that strong field enhancement from plasmonic metasurfaces enables the use of MEIRS as a highly informative analytic technique for real-time monitoring of cells. By exposing live cells cultured on a plasmonic metasurface to chemical compounds, we show that MEIRS can be used as a label-free phenotypic assay for detecting multiple cellular responses to external stimuli: changes in cell morphology, adhesion, and lipid composition of the cellular membrane, as well as intracellular signaling. Using a focal plane array detection system, we show that MEIRS also enables spectro-chemical imaging at the single-cell level. The described metasurface-based all-optical sensor opens the way to a scalable, high-throughput spectroscopic assay for live cells.

Applications of Converged Various Forces for Detection of Biomolecules and Novelty of Dielectrophoretic Force in the Applications

Since separation of target biomolecules is a crucial step for highly sensitive and selective detection of biomolecules, hence, various technologies have been applied to separate biomolecules, such as deoxyribonucleic acid (DNA), protein, exosome, virus, etc. Among the various technologies, dielectrophoresis (DEP) has the significant advantage that the force can provide two different types of forces, attractive and repulsive DEP force, through simple adjustment in frequency or structure of microfluidic chips. Therefore, in this review, we focused on separation technologies based on DEP force and classified various separation technologies. First, the importance of biomolecules, general separation methods and various forces including DEP, electrophoresis (EP), electrothermal flow (ETF), electroosmosis (EO), magnetophoresis, acoustophoresis (ACP), hydrodynamic, etc., was described. Then, separating technologies applying only a single DEP force and dual force, moreover, applying other forces simultaneously with DEP force were categorized. In addition, advanced technologies applying more than two different kinds of forces, namely complex force, were introduced. Overall, we critically reviewed the state-of-the-art of converged various forces for detection of biomolecules with novelty of DEP.