Modulation of Cellular Reactivity for Enhanced Cell-Based Biosensing.

Cell-based sensing platforms provide functional information on cellular effects of bioactive or toxic compounds in a sample. Current challenges concern the rather extended length of the assays as well as their limited reproducibility and sensitivity. We present a biosensing method capable of appraising, on a short time scale and with exquisite sensitivity, the occurrence and the magnitude of cellular alterations induced by low levels of a bioactive/toxic compound. Our method is based on integrating optogenetic control of non-electrogenic human cells, modified to express light sensitive protein channels, into a non-invasive electro-optical analytical platform enabling quantitative assessment of the stimulus dependent, dynamical cellular response. Our system exploits the interplay between optogenetic stimulation and time lapse fast impedance assays in boosting the platform sensitivity when exposing cells to a model exogenous stimulus, under both static and flow conditions. The proposed optogenetically modulated cell-based sensing platform is suitable for in field applications and provides a new paradigm for impedance-based sensing.

Quantitative assessment of specific carbonic anhydrase inhibitors effect on hypoxic cells using electrical impedance assays.

Carbonic anhydrase IX (CA IX) is an important orchestrator of hypoxic tumour environment, associated with tumour progression, high incidence of metastasis and poor response to therapy. Due to its tumour specificity and involvement in associated pathological processes: tumourigenesis, angiogenesis, inhibiting CA IX enzymatic activity has become a valid therapeutic option. Dynamic cell-based biosensing platforms can complement cell-free and end-point analyses and supports the process of design and selection of potent and selective inhibitors. In this context, we assess the effectiveness of recently emerged CA IX inhibitors (sulphonamides and sulphocoumarins) and their antitumour potential using an electrical impedance spectroscopy biosensing platform. The analysis allows discriminating between the inhibitory capacities of the compounds and their inhibition mechanisms. Microscopy and biochemical assays complemented the analysis and validated impedance findings establishing a powerful biosensing tool for the evaluation of carbonic anhydrase inhibitors potency, effective for the screening and design of anticancer pharmacological agents.

Complementarity of EIS and SPR to reveal specific and nonspecific binding when interrogating a model bioaffinity sensor; perspective offered by plasmonic based EIS.

The present work compares the responses of a model bioaffinity sensor based on a dielectric functionalization layer, in terms of specific and nonspecific binding, when interrogated simultaneously by Surface Plasmon Resonance (SPR), non-Faradaic Electrochemical Impedance Spectroscopy (EIS), and Plasmonic based-EIS (P-EIS). While biorecognition events triggered a sensitive SPR signal, the related EIS response was rather negligible. Contrarily, even a limited nonspecific adsorption onto the surface of the metallic electrode, allowed by the intrinsic imperfect compactness of the functionalization layers, was signaled by EIS and not by SPR. The source of this finding has been addressed from both theoretical and experimental perspectives, demonstrating that EIS signals are mainly sensitive to adsorptions that alter the current pathway through defects of the functionalization layer exposing the electrode. These observations are of importance for those developing biosensors analyzed by SPR, EIS, or the novel combination of the two methods (P-EIS). A possible application of the observed complementarity of the two methods, namely assessment of sample purity in respect to a target analyte is highlighted. Moreover, the possibility of false-positive EIS responses (determined by nonspecific binding) when assessing samples containing complex matrices or consisting of small molecular weight analytes is emphasized.

Progress in the Optical Sensing of Cardiac Biomarkers.

Biomarkers play key roles in the diagnosis, risk assessment, treatment and supervision of cardiovascular diseases (CVD). Optical biosensors and assays are valuable analytical tools answering the need for fast and reliable measurements of biomarker levels. This review presents a survey of recent literature with a focus on the past 5 years. The data indicate continuing trends towards multiplexed, simpler, cheaper, faster and innovative sensing while newer tendencies concern minimizing the sample volume or using alternative sampling matrices such as saliva for less invasive assays. Utilizing the enzyme-mimicking activity of nanomaterials gained ground in comparison to their more traditional roles as signaling probes, immobilization supports for biomolecules and for signal amplification. The growing use of aptamers as replacements for antibodies prompted emerging applications of DNA amplification and editing techniques. Optical biosensors and assays were tested with larger sets of clinical samples and compared with the current standard methods. The ambitious goals on the horizon for CVD testing include the discovery and determination of relevant biomarkers with the help of artificial intelligence, more stable specific recognition elements for biomarkers and fast, cheap readers and disposable tests to facilitate rapid testing at home. As the field is progressing at an impressive pace, the opportunities for biosensors in the optical sensing of CVD biomarkers remain significant.

Fast impedimetric immunosensing of IgGs associated with peanut and hazelnut allergens.

Food allergies trigger a variety of clinical adverse symptoms and clinical evidence suggests that the presence of food allergy-related IgG can be helpful in the diagnosis when analyzed at the peptide-epitope level. To validate and select the peptides based on their specificity toward hazelnut or peanut epitopes, the authors of this study developed a silicon-based microchip coupled with click-chemistry bound peptides identified by the Fraunhofer Institute for Cell Therapy and Immunology. Peptides related to hazelnut and peanut allergies were identified and used to develop a silicon-based microchip. Peptides were coupled with click-chemistry to the sensor surface. The immunosensor was developed by electrografting diazotized amino phenylacetic acid and subsequently, dibenzocyclooctyne-amine (DBCO-NH2) was used as click-chemistry to allow coupling of the peptides with a C-terminal linker and azide structure. Energy-dispersive X-ray spectroscopy, electrochemical impedance spectroscopy (EIS), and fluorescence microscopy techniques have been used to analyze the bio-functionalization of the developed electrode. The peptide-epitope recognition was studied for seven allergen-derived peptides. The electrochemical responses were studied with sera from rabbits immunized with hazelnut and peanut powder. The microchips functionalized with the chosen peptides (peanut peptides T12 and EO13 and hazelnut peptides S4 and EO14 with an RSD of 4%, 3%, 9%, and 1% respectively) demonstrated their ability to specifically detect prevalent anti-nut related IgGs in rabbit sera in a range of dilutions from 1:500000 (0.0002%) until 1:50000 (0.002%). In addition, the other peptides showed promising differentiation abilities which can be further studied to perform multivariable detection fingerprint of anti-allergens in blood sera.

Real Time SPR Assessment of the Structural Changes of Adaptive Dynamic Constitutional Frameworks as a New Route for Sensing.

Cross linked gold-dynamic constitutional frameworks (DCFs) are functional materials of potential relevance for biosensing applications, given their adaptivity and high responsivity against various external stimuli (such as pH, temperature) or specific interactions with biomolecules (enzymes or DNA) via internal constitutional dynamics. However, characterization and assessment of their dynamic conformational changes in response to external stimuli has never been reported. This study proves the capability of Surface Plasmon Resonance (SPR) assays to analyse the adaptive structural modulation of a functional matrix encompassing 3D gold-dynamic constitutional frameworks (Au-DCFs) when exposed to pH variations, as external stimuli. We analyse Au-DCFs formed from Au nanoparticles, (AuNP) connected through constitutionally dynamic polymers, dynamers, with multiple functionalities. For increased generality of this proof-of-concept assay, Au-DCFs, involving DCFs designed from 1,3,5-benzene-tricarbaldehyde (BTA) connecting centres and polyethylene glycol (PEG) connectors, are covalently attached to standard SPR sensing chips (Au nanolayers, carboxyl terminated or with carboxymethyl dextran, CMD top-layer) and analysed using state-of-the art SPR instrumentation. The SPR effects of the distance from the Au-DCFs matrix to the Au nanolayer of the sensing chip, as well as of Au-DCFs thickness were investigated. This study reveals the SPR response, augmented by the AuNP, to the conformational change, i.e., shrinkage, of the dynamer and AuNP matrix when decreasing the pH, and provides an unexplored insight into the sensing applicability of SPR real-time analysis of adaptive functional materials.

Advanced Optogenetic-Based Biosensing and Related Biomaterials.

The ability to stimulate mammalian cells with light, brought along by optogenetic control, has significantly broadened our understanding of electrically excitable tissues. Backed by advanced (bio)materials, it has recently paved the way towards novel biosensing concepts supporting bio-analytics applications transversal to the main biomedical stream. The advancements concerning enabling biomaterials and related novel biosensing concepts involving optogenetics are reviewed with particular focus on the use of engineered cells for cell-based sensing platforms and the available toolbox (from mere actuators and reporters to novel multifunctional opto-chemogenetic tools) for optogenetic-enabled real-time cellular diagnostics and biosensor development. The key advantages of these modified cell-based biosensors concern both significantly faster (minutes instead of hours) and higher sensitivity detection of low concentrations of bioactive/toxic analytes (below the threshold concentrations in classical cellular sensors) as well as improved standardization as warranted by unified analytic platforms. These novel multimodal functional electro-optical label-free assays are reviewed among the key elements for optogenetic-based biosensing standardization. This focused review is a potential guide for materials researchers interested in biosensing based on light-responsive biomaterials and related analytic tools.

High-resolution impedance mapping using electrically activated quantitative phase imaging.

Retrieving electrical impedance maps at the nanoscale rapidly via nondestructive inspection with a high signal-to-noise ratio is an unmet need, likely to impact various applications from biomedicine to energy conversion. In this study, we develop a multimodal functional imaging instrument that is characterized by the dual capability of impedance mapping and phase quantitation, high spatial resolution, and low temporal noise. To achieve this, we advance a quantitative phase imaging system, referred to as epi-magnified image spatial spectrum microscopy combined with electrical actuation, to provide complementary maps of the optical path and electrical impedance. We demonstrate our system with high-resolution maps of optical path differences and electrical impedance variations that can distinguish nanosized, semi-transparent, structured coatings involving two materials with relatively similar electrical properties. We map heterogeneous interfaces corresponding to an indium tin oxide layer exposed by holes with diameters as small as ~550 nm in a titanium (dioxide) over-layer deposited on a glass support. We show that electrical modulation during the phase imaging of a macro-electrode is decisive for retrieving electrical impedance distributions with submicron spatial resolution and beyond the limitations of electrode-based technologies (surface or scanning technologies). The findings, which are substantiated by a theoretical model that fits the experimental data very well enable achieving electro-optical maps with high spatial and temporal resolutions. The virtues and limitations of the novel optoelectrochemical method that provides grounds for a wider range of electrically modulated optical methods for measuring the electric field locally are critically discussed.

Cellular sensing platform with enhanced sensitivity based on optogenetic modulation of cell homeostasis.

We demonstrate a new biosensing concept with impact on the development of rapid, point of need cell based sensing with boosted sensitivity and wide relevance for bioanalysis. It involves optogenetic stimulation of cells stably transfected to express light sensitive protein channels for optical control of membrane potential and of ion homeostasis. Time-lapse impedance measurements are used to reveal cell dynamics changes encompassing cellular responses to bioactive stimuli and optically induced homeostasis disturbances. We prove that light driven perturbations of cell membrane potential induce homeostatic reactions and modulate transduction mechanisms that amplify cellular response to bioactive compounds. This allows cell based biosensors to respond more rapidly and sensitively to low concentrations of bioactive/toxic analytes: statistically relevant impedance changes are recorded in less than 30 min, in comparison with >8 h in the best alternative reported tests for the same low concentration (e.g. a concentration of 25 µM CdCl2, lower than the threshold concentration in classical cellular sensors). Comparative analysis of model bioactive/toxic compounds (ouabain and CdCl2) demonstrates that cellular reactivity can be boosted by light driven perturbations of cellular homeostasis and that this biosensing concept is able to discriminate analytes with different modes of action (i.e. CdCl2 toxicity versus ion pump inhibition by ouabain), a significant advance against state of the art cell based sensors.

High spatial resolution electrochemical biosensing using reflected light microscopy.

If the analyte does not only change the electrochemical but also the optical properties of the electrode/solution interface, the spatial resolution of an electrochemical sensor can be substantially enhanced by combining the electrochemical sensor with optical microscopy. In order to demonstrate this, electrochemical biosensors for the detection of hydrogen peroxide and glucose were developed by drop casting enzyme and redox polymer mixtures onto planar, optically transparent electrodes. These biosensors generate current signals proportional to the analyte concentration via a reaction sequence which ultimately changes the oxidation state of the redox polymer. Images of the interface of these biosensors were acquired using bright field reflected light microscopy (BFRLM). Analysis showed that the intensity of these images is higher when the redox polymer is oxidized than when it is reduced. It also revealed that the time needed for the redox polymer to change oxidation state can be assayed optically and is dependent on the concentration of the analyte. By combining the biosensor for hydrogen peroxide detection with BFRLM, it was possible to determine hydrogen peroxide in concentrations as low as 12.5 µM with a spatial resolution of 12 µm × 12 µm, without the need for the fabrication of microelectrodes of these dimensions.

High speed CMOS acquisition system based on FPGA embedded image processing for electro-optical measurements.

Electro-optical measurements, i.e., optical waveguides and plasmonic based electrochemical impedance spectroscopy (P-EIS), are based on the sensitive dependence of refractive index of electro-optical sensors on surface charge density, modulated by an AC electrical field applied to the sensor surface. Recently, P-EIS has emerged as a new analytical tool that can resolve local impedance with high, optical spatial resolution, without using microelectrodes. This study describes a high speed image acquisition and processing system for electro-optical measurements, based on a high speed complementary metal-oxide semiconductor (CMOS) sensor and a field-programmable gate array (FPGA) board. The FPGA is used to configure CMOS parameters, as well as to receive and locally process the acquired images by performing Fourier analysis for each pixel, deriving the real and imaginary parts of the Fourier coefficients for the AC field frequencies. An AC field generator, for single or multi-sine signals, is synchronized with the high speed acquisition system for phase measurements. The system was successfully used for real-time angle-resolved electro-plasmonic measurements from 30 Hz up to 10 kHz, providing results consistent to ones obtained by a conventional electrical impedance approach. The system was able to detect amplitude variations with a relative variation of ±1%, even for rather low sampling rates per period (i.e., 8 samples per period). The PC (personal computer) acquisition and control software allows synchronized acquisition for multiple FPGA boards, making it also suitable for simultaneous angle-resolved P-EIS imaging.

Biosensing Based on Magneto-Optical Surface Plasmon Resonance.

In spite of the high analytic potential of Magneto Optical Surface Plasmon Resonance (MOSPR) assays, their applicability to biosensing has been limited due to significant chip stability issues. We present novel solutions to surpass current limitations of MOSPR sensing assays, based on innovative chip structure, tailored measurements and improved data analysis methods. The structure of the chip is modified to contain a thin layer of Co-Au alloy instead of successive layers of homogenous metals with magnetic and plasmonic properties, as currently used. This new approach presents improved plasmonic and magnetic properties, yet a structural stability similar to standard Au-SPR chips, allowing for bioaffinity assays in saline solutions. Moreover, using a custom-designed measurement configuration that allows the acquisition of the SPR curve, i.e., the reflectivity measured at multiple angles of incidence, instead of the reflectivity value at a single-incidence angle, a high signal-to-noise ratio is achieved, suitable for detection of minute analyte concentrations. The proposed structure of the MOSPR sensing chip and the procedure of data analysis allow for long time assessment in liquid media, a significant advancement over existing MOSPR chips, and confirm the MOSPR increased sensitivity over standard SPR analyses.

Magneto-plasmonic biosensor with enhanced analytical response and stability.

We present novel solutions to surpass current analytic limitations of Magneto-Optical Surface Plasmon Resonance (MOSPR) assays, concerning both the chip structure and the method for data analysis. The structure of the chip is modified to contain a thin layer of Co-Au alloy instead of successive layers of homogeneous metals, as currently used. This alloy presents improved plasmonic and magnetic properties, yet a structural stability similar to Au-SPR chips, allowing for bioaffinity assays in saline solutions. Analyzing the whole reflectivity curve at multiple angles of incidence instead of the reflectivity value at a single incidence angle provides a high signal-to-noise ratio suitable for detection of minute analyte concentrations. Based on assessment of solutions with known refractive indices as well as of a model biomolecular interaction (i.e. IgG-AntiIgG) we demonstrate that the proposed structure of the MOSPR sensing chip and the procedure of data analysis allows for long-time assessment in liquid media with increased sensitivity over standard SPR analyses.

Label free sensing platform for amyloid fibrils effect on living cells.

This study presents a multiparametric label-free analysis gathering surface plasmon resonance (SPR) and electrical impedance spectroscopy (EIS) for monitoring the progress of a model epithelial cell culture (Madin Darbey Canine Kidney - MDCK) exposed to a peptide with high bio-medical relevance, amyloid ß (Aß42). The approach surpasses the limitations in using the SPR angle for analyzing confluent cell monolayers and proposes a novel quantitative analysis of the SPR dip combined with advanced EIS as a tool for dynamic cell assessment. Long, up to 48h time series of EIS and SPR data reveal a biphasic cellular response upon Aß42 exposure corresponding to changes in cell-substrate adherence, cell-cell tightening or cytoskeletal remodeling. The equivalent circuit used for fitting the EIS spectra provided substantiation of SPR analysis on the progress of cell adhesion as well as insight on dynamics of cell-cell junction. Complementary endpoint assays: western blot analysis and atomic force microscopy experiments have been performed for validation. The proposed label free sensing of nonlethal effect of model amyloid protein at cellular level provides enhanced resolution on cell-surface and cell-cell interactions modulated by membrane related protein apparatus, applicable as well to other adherent cell types and amyloid compounds.

Quality assessment of SPR sensor chips; case study on L1 chips.

Surface quality of the Surface Plasmon Resonance (SPR) chips is a major limiting issue in most SPR analyses, even more for supported lipid membranes experiments, where both the organization of the lipid matrix and the subsequent incorporation of the target molecule depend on the surface quality. A novel quantitative method to characterize the quality of SPR sensors chips is described for L1 chips subject to formation of lipid films, injection of membrane disrupting compounds, followed by appropriate regeneration procedures. The method consists in analysis of the SPR reflectivity curves for several standard solutions (e.g. PBS, HEPES or deionized water). This analysis reveals the decline of sensor surface as a function of the number of experimental cycles (consisting in biosensing assay and regeneration step) and enables active control of surface regeneration for enhanced reproducibility. We demonstrate that quantitative evaluation of the changes in reflectivity curves (shape of the SPR dip) and of the slope of the calibration curve provides a rapid and effective procedure for surface quality assessment. Whereas the method was tested on L1 SPR sensors chips, we stress on its amenability to assess the quality of other types of SPR chips, as well.

Assessment of pathogenic bacteria using periodic actuation.

A new analytical platform for the assessment of pathogenic bacteria is presented. It is based on a robust technology which is able to amplify the signal to noise ratio providing fast and sensitive detection of target pathogenic bacteria. The system uses a custom made AC electrical impedance analyser to measure, using a lab on a chip platform, the oscillations of magnetically labelled analytes when applying a periodic magnetic field. The concentration of pathogenic Escherichia coli O157:H7 chosen as bacterial model was determined based on the amplitude of the electrical impedance oscillations at a selected AC frequency. The analytical platform provides a limit of detection of 10(2) cells ml(-1), has a fast analysis time, and is amenable for the detection of other target cells. The system has simple design suitable for portability and automated operation.

Simultaneous impedimetric and amperometric interrogation of renal cells exposed to a calculus-forming salt.

The complexity of the cellular response, induced even by the simplest experimental stimulus, requires an increased number of cellular parameters to be simultaneously monitored. An all electrochemical system allowing the simultaneous and real-time monitoring of both cell adherence and superoxide release into the extracellular space was developed to address this challenge. Cell adherence (to neighboring cells and to substrate) was monitored using non-faradaic impedance spectroscopy while the superoxide release was monitored using a cytochrome c-based amperometric biosensor. The system was used to observe for the first time how these two cellular parameters are changing in real-time for renal cells exposed to calcium oxalate, a calculus-forming salt. It was discovered that calcium oxalate crystals decrease cell adherence and in the same time induce oxidative stress by an overproduction of superoxide. Subconfluent cells, without fully developed tight junctions, appear to be more vulnerable than confluent cells with tight junctions indicating the important protective role of these junctions.

A novel low-cost and easy to develop functionalization platform. Case study: aptamer-based detection of thrombin by surface plasmon resonance.

A novel low-cost platform to assess biomolecular interactions was investigated using surface plasmon resonance and an aptamer-based assay for thrombin detection. Gold SPR surface functionalized with a carboxylated cross-linked BSA film (cBSA) and commercially available carboxymethylated dextran chip (CM5) were used as immobilization platforms for the thrombin binding aptamer. The high end commercial instrument Biacore 3000 and a custom made FIA set-up involving TI Spreeta sensor (TSPR2K23) were used to assess different concentrations of thrombin within the range 0.1-150 nM both in buffer and in a complex matrix (plasma) using the obtained aptasensors. Based on data derived from both CM5 and cBSA platforms, the cBSA aptasensor exhibited good selectivity, stability and regeneration ability, both in buffer and in complex matrices (plasma), comparable with CM5.

Sensing based on assessment of non-monotonous effect determined by target analyte: case study on pore-forming compounds.

A new and exciting biosensing avenue based on assessment of the non-monotonous, concentration dependent effect of pore formation is discussed. A novel kinetic model is advanced to relate surface plasmon resonance (SPR) data with actual concentrations of interacting partners. Lipid modified L1 sensor chip provide the accessible platform for SPR exploration of peptide-membrane interaction, with POPC and melittin as model systems. We show that quantitative assessment of the interaction between an antimicrobial peptide and lipid modified sensors is capable to provide both sensing avenues and detailed mechanistic insights into effects of pore-forming compounds. The proposed model combined with appropriate design of the experimental protocol adds a new depth to the classic SPR investigation of peptide-lipid interaction offering a quantitative platform for detection, improved understanding of the manifold facets of the interaction and for supporting the controlled design of novel antimicrobial compounds. This biosensing approach can be applied to an entire set of pore-forming compounds including antimicrobial peptides and exo-toxins.