Label-free Optical Detection Research Articles

(Keynote) Blood, Sweat and Tears... – Sensing of Human Performance Biomarkers in Bodily Fluids Using Point-of-Use Devices

Hormones produced by glands in the endocrine system and neurotransmitters produced by the nervous system control many bodily functions. The concentrations of these molecules in the body are an indication of its state, hence the use of the term biomarker. Excess concentrations of biomarkers, such as cortisol, serotonin, epinephrine, dopamine, are released by the body in response to a variety of conditions - emotional state (euphoria, stress), disease, etc. The development of simple, low-cost modalities for point-of-use (PoU) measurements of biomarkers levels in various bodily fluids (blood, urine, sweat, saliva) as opposed to conventional hospital or lab settings is receiving increasing attention. The presentation starts with a review of the basic properties of primary stress-related biomarkers: origin in the body (i.e. if they are produced as hormones, neurotransmitters or both), chemical composition, molecular weight (small/medium size molecules and polymers, ranging from ~100Da to ~100kDa), hydro- or lipo-philic nature. Next, a review of the published literature is presented regarding the concentration of these biomarkers found in several bodily fluids that can serve as the medium for determination of the condition of the subject: blood, urine, saliva, sweat and, to a lesser degree, interstitial tissue fluid. The concentration of various biomarkers in most fluids covers a range of 5-6 orders of magnitude, from 100s of ng/mL (~1µM) down to a few pg/mL (sub 1pM). Mechanisms and materials for point-of-use biomarker sensors are summarized and key properties are reviewed. Illustrative examples from the literature are discussed for several sensor device categories, including lateral flow (immuno)assay devices and microfluidic devices. Selected methods for detecting these biomarkers are reviewed, including antibody- and aptamer-based colorimetric assays, electrochemical and optical detection. Finally, the presentation outlines key challenges of the field and provides a look ahead to future prospects.A. J. Steckl and P. Ray, “A Review of Stress Biomarkers Concentrations in Bodily Fluids and Their Point-of-Use Detection”, ACS Sensors, 3 (10), 2025-2044, Sept 2018 (10.1021/acssensors.8b00726)V. Venkatraman and A. J. Steckl, “Quantitative Detection in Lateral Flow Immunoassay Using Integrated Organic Optoelectronics”, IEEE Sensors, 17 (24), 8343, Dec 2017 (10.1109/JSEN.2017.2764178 ).P. Ray and A. J. Steckl, “Label-Free Optical Detection of Multiple Stress Biomarkers in Sweat, Plasma, Urine and Saliva”, ACS Sensors 4, 2019. (10.1021/acssensors.9b00301)S. Dalirirad and A. J. Steckl, “Aptamer-Based Lateral Flow Assay for Point of Care Cortisol Detection in Sweat”, Sensors & Actuators 283, 79-86, Jan 2019 (https://doi.org/10.1016/j.snb.2018.11.161). Figure 1

A Label-Free Diamond Microfluidic DNA Sensor Based on Active Nitrogen-Vacancy Center Charge State Control.

We propose a label-free biosensor concept based on the charge state manipulation of nitrogen-vacancy (NV) quantum color centers in diamond, combined with an electrochemical microfluidic flow cell sensor, constructed on boron-doped diamond. This device can be set at a defined electrochemical potential, locking onto the particular chemical reaction, whilst the NV center provides the sensing function. The NV charge state occupation is initially prepared by applying a bias voltage on a gate electrode and then subsequently altered by exposure to detected charged molecules. We demonstrate the functionality of the device by performing label-free optical detection of DNA molecules. In this experiment, a monolayer of strongly cationic charged polymer polyethylenimine is used to shift the charge state of near surface NV centers from negatively charged NV- to neutral NV0 or dark positively charged NV+. Immobilization of negatively charged DNA molecules on the surface of the sensor restores the NV centers charge state back to the negatively charged NV-, which is detected using confocal photoluminescence microscopy. Biochemical reactions in the microfluidic channel are characterized by electrochemical impedance spectroscopy. The use of the developed electrochemical device can also be extended to nuclear magnetic resonance spin sensing.

One-photon excited photoluminescence of gold nanospheres and its application in prostate specific antigen detection via fluorescence correlation spectroscopy (FCS)

Gold nanoparticles are known to exhibit appealing intrinsic plasmon-modulated photoluminescence (PL) properties which can be explored in various fluorescence-based sensing applications. In this paper, we evaluate the PL of different-sized gold nanospheres (AuNSs) under one-photon excitation (1PE) and develop a sensitive homogeneous immunoassay for the detection of prostate specific antigen (PSA) in colloidal suspension via fluorescence correlation spectroscopy (FCS). The 1PE PL of AuNSs of three different sizes are evaluated in solution phase under excitation at 405 nm via steady-state fluorescence spectroscopy measurements, while FCS analysis emphasizes the feasibility of using 1PE PL properties to monitor their diffusion behavior. Fluorescence lifetime imaging microscopy (FLIM) assays coupled with PL spectral profile analysis performed on single-particles-like structures conform the plasmonic origin of the detected PL and validate their potential of synthesized AuNSs as fluorescent probes in bioimaging and bioassays. Finally, to the best of our knowledge, we provide the first demonstration of the successful use of the 1PE PL of the synthesized AuNSs as probes for the FCS-based one-step label-free sensitive optical detection of PSA biomarker. The approach consisting in monitoring the diffusion of the AuNSs-oligomers induced by the interaction of anti-PSA-conjugated AuNSs with PSA molecules is successfully validated for the detection of PSA levels as low as 4.4 ng/ml in solution. Considering that the development of rapid, efficient and label-free biosensing methods is of continuous interest nowadays, we are confident that our results may have a strong impact on medicine towards more efficient, sensitive and reliable diagnosis.

Three-Dimensional Interfacing of Cells with Hierarchical Silicon Nano/Microstructures for Midinfrared Interrogation of In Situ Captured Proteins.

Label-free optical detection of biomolecules is currently limited by a lack of specificity rather than sensitivity. To exploit the much more characteristic refractive index dispersion in the mid-infrared (IR) regime, we have engineered three-dimensional IR-resonant silicon micropillar arrays (Si-MPAs) for protein sensing. By exploiting the unique hierarchical nano- and microstructured design of these Si-MPAs attained by CMOS-compatible silicon-based microfabrication processes, we achieved an optimized interrogation of surface protein binding. Based on spatially resolved surface functionalization, we demonstrate controlled three-dimensional interfacing of mammalian cells with Si-MPAs. Spatially controlled surface functionalization for site-specific protein immobilization enabled efficient targeting of soluble and membrane proteins into sensing hotspots directly from cells cultured on Si-MPAs. Protein binding to Si-MPA hotspots at submonolayer level was unambiguously detected by conventional Fourier transform IR spectroscopy. The compatibility with cost-effective CMOS-based microfabrication techniques readily allows integration of this novel IR transducer into fully fledged bioanalytical microdevices for selective and sensitive protein sensing.

Label-Free Homogeneous microRNA Detection in Cell Culture Medium Based on Graphene Oxide and Specific Fluorescence Quenching.

Label-free homogeneous optical detection of low concentration of oligonucleotides using graphene oxide in complex solutions containing proteins remains difficult. We used a colloidal graphene oxide (GO) as a fluorescent probe quencher to detect microRNA-21 spiked-in cell culture medium, overcoming previously reported problematic aspects of protein interference with graphene oxide. We used a “turn off” assay for specific quenching-based detection of oligo DNA-microRNA hybridization in solution. A fluorescein conjugated 30-mer single-stranded DNA (ssDNA) probe was combined with a complementary synthetic microRNA (18 nucleotides) target. The probe-target hybridization was detected by specific quenching due to photoinduced electron transfer (PET). On the next step, GO captures and quenches the unhybridized probe by fluorescence resonance energy transfer (FRET) in the presence of cell culture medium supplemented with platelet lysate, 0.1% sodium dodecyl sulfate (SDS), 0.1% Triton X-100 and 50% formamide. This resulted in sensitive measurement of the specific probe-target complexes remaining in solution. The detection is linear in the range of 1 nM and 8 nM in a single 100 μL total volume assay sample containing 25% cell culture medium supplemented with platelet lysate. We highlight a general approach that may be adopted for microRNA target detection within complex physiological media.

An integrated electro-optical biosensor system for rapid, low-cost detection of bacteria

In medical treatment, the detection of pathogens at an early stage of diseases is a key step to set up an appropriate diagnosis. To reach this goal, several techniques have been elaborated for point-of-care diagnostic applications. One of the state-of-the art methods is the application of biosensor devices. Label-free versions of them ensure an appropriate detection of pathogens from fluid samples by their relative sensitivity, rapidity and portability, thus offering a feasible and affordable alternative to the traditional diagnostic techniques. The aim of the present study is to fulfill these requirements with a cheap construction of an electro-optical biosensor, for application as a rapid test in clinical diagnostics. Hence, an integrated microsystem consisting of dielectrophoretic surface-electrodes, a rib waveguide and a microfluidic channel was created for label-free optical detection of bacteria from fluid samples. To model the efficiency of the sensor, we carried out quantitative measurements by observing the light scattered by living Escherichia coli cells located in the vicinity of the waveguide. A significant change in the scattered light pattern was observed even when objectives of moderate magnification (x10, x4.7) were used, implying that such type of sensing of the cells can be achieved by low-cost cameras, as well. The optimal frequency utilized in the process of dielectrophoretic cell-collecting was also established. With this novel system, a detection limit of ca. 102CFU × mL−1 was achieved, which is relevant to characteristic pathogen concentrations in body fluids, e.g., urine. Our further plan is to utilize this cell-gathering method in other, highly sensitive integrated optical sensor constructions, as well. The working principle of this dielectrophoretically enhanced detection of Escherichia coli cells from their suspensions gives us a low-cost and rapid-sensing alternative to routinely used, but time- and money-consuming other methods. Hence, we expect it to be readily applicable in point-of-care diagnostics as a basis of rapid tests to identify general pathogens from various body fluids.

A Label-Free Optical Detection of Pathogens in Isopropanol as a First Step towards Real-Time Infection Prevention

The detection of pathogens is a major public health issue. Every year, thousands of people die because of nosocomial infections. It is therefore important to be able to detect possible outbreaks as early as possible, especially in the hospital environment. Various pathogen detection techniques have already been demonstrated. However, most of them require expensive and specific equipment, and/or complex protocols, which, most of the time, involve biochemical reaction and labelling steps. In this paper, a new method that combines microscopic imaging and machine learning is described. The main benefits of this approach are to be low-cost, label-free and easy to integrate in any suitable medical device, such as hand hygiene dispensers. The suitability of this pathogen detection method is validated using four bacteria, both in PBS (Phosphate Buffered Saline) and in isopropanol. In particular, we demonstrated an efficient pathogenic detection that is sensible to changes in the composition of a mixture of pathogens, even in alcohol-based solutions.

A lectin-coupled porous silicon-based biosensor: label-free optical detection of bacteria in a real-time mode

Accuracy and speed of detection, along with technical and instrumental simplicity, are indispensable for the bacterial detection methods. Porous silicon (PSi) has unique optical and chemical properties which makes it a good candidate for biosensing applications. On the other hand, lectins have specific carbohydrate-binding properties and are inexpensive compared to popular antibodies. We propose a lectin-conjugated PSi-based biosensor for label-free and real-time detection of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) by reflectometric interference Fourier transform spectroscopy (RIFTS). We modified meso-PSiO2 (10–40 nm pore diameter) with three lectins of ConA (Concanavalin A), WGA (Wheat Germ Agglutinin), and UEA (Ulex europaeus agglutinin) with various carbohydrate specificities, as bioreceptor. The results showed that ConA and WGA have the highest binding affinity for E. coli and S. aureus respectively and hence can effectively detect them. This was confirmed by 6.8% and 7.8% decrease in peak amplitude of fast Fourier transform (FFT) spectra (at 105 cells mL−1 concentration). A limit of detection (LOD) of about 103 cells mL−1 and a linear response range of 103 to 105 cells mL−1 were observed for both ConA-E. coli and WGA-S. aureus interaction platforms that are comparable to the other reports in the literature. Dissimilar response patterns among lectins can be attributed to the different bacterial cell wall structures. Further assessments were carried out by applying the biosensor for the detection of Klebsiella aerogenes and Bacillus subtilis bacteria. The overall obtained results reinforced the conjecture that the WGA and ConA have a stronger interaction with Gram-positive and Gram-negative bacteria, respectively. Therefore, it seems that specific lectins can be suggested for bacterial Gram-typing or even serotyping. These observations were confirmed by the principal component analysis (PCA) model.

Mass Transfer Limitations of Porous Silicon-Based Biosensors for Protein Detection.

Porous silicon (PSi) thin films have been widely studied for biosensing applications, enabling label-free optical detection of numerous targets. The large surface area of these biosensors has been commonly recognized as one of the main advantages of the PSi nanostructure. However, in practice, without application of signal amplification strategies, PSi-based biosensors suffer from limited sensitivity, compared to planar counterparts. Using a theoretical model, which describes the complex mass transport phenomena and reaction kinetics in these porous nanomaterials, we reveal that the interrelated effect of bulk and hindered diffusion is the main limiting factor of PSi-based biosensors. Thus, without significantly accelerating the mass transport to and within the nanostructure, the target capture performance of these biosensors would be comparable, regardless of the nature of the capture probe–target pair. We use our model to investigate the effect of various structural and biosensor characteristics on the capture performance of such biosensors and suggest rules of thumb for their optimization.

Ultrasensitive and rapid detection of malaria using graphene-enhanced surface plasmon resonance

Extraordinary optical, electrical and chemical properties of 2D materials have potential to be useful for quick and sensitive detection of pathological diseases. One important example is malaria disease that can progress rapidly and cause death within days. Therefore, fast, accurate and cost-effective malaria diagnosis available at the point of care is urgently needed to facilitate precise treatment. Here we report rapid and highly sensitive malaria detection with an inexpensive graphene-protected copper surface plasmon resonance biosensor. Using phase sensitive surface plasmon resonance technique and a graphene functionalization protocol for attaching end-tethered DNA probes that were complementary to a malaria specific DNA target, we were able to significantly improve the detection limit of the malarial plasmodium parasite. The phase sensitivity of our graphene-enhanced sensors exceeds by two orders of magnitude the sensitivity of analogous optical biosensors. This enhanced sensitivity could provide means to detect low copy number bacterial infectious agents and to associate dormant bacterial populations with chronic inflammatory diseases using simple label-free optical detection.

Label-free optical detection of bioelectric potentials using electrochromic thin films

Understanding how a network of interconnected neurons receives, stores, and processes information in the human brain is one of the outstanding scientific challenges of our time. The ability to reliably detect neuroelectric activities is essential to addressing this challenge. Optical recording using voltage-sensitive fluorescent probes has provided unprecedented flexibility for choosing regions of interest in recording neuronal activities. However, when recording at a high frame rate such as 500 to 1,000 Hz, fluorescence-based voltage sensors often suffer from photobleaching and phototoxicity, which limit the recording duration. Here, we report an approach called electrochromic optical recording (ECORE) that achieves label-free optical recording of spontaneous neuroelectrical activities. ECORE utilizes the electrochromism of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) thin films, whose optical absorption can be modulated by an applied voltage. Being based on optical reflection instead of fluorescence, ECORE offers the flexibility of an optical probe without suffering from photobleaching or phototoxicity. Using ECORE, we optically recorded spontaneous action potentials in cardiomyocytes, cultured hippocampal and dorsal root ganglion neurons, and brain slices. With minimal perturbation to cells, ECORE allows long-term optical recording over multiple days.

(Electronics and Photonics Division Award Address) Let There be Light... – Research at the Intersection of Electronics and Photonics

Electronics and photonics have a long and successful history of combining to provide many devices and products that are at the heart of the modern technological world, ranging form solar cells to lasers to digital displays. In this presentation I will first give a brief historical overview of the interplay and interdependence of electronics and photonics. The properties and effects that are commonly exploited in opto/electronics fall primarily in two areas: (a) the effect of incident light (or more generally radiation) due to optical absorption/transmission/reflection and the conversion of photons to electrical charge that can be collected; (b) the injection of electrons & holes leading to the emission of photons. Then I will review selected examples from my research in optoelectronic materials and devices in more or less chronological order over the past 40+ years. An early example [1] in the light-to-charge was integration of infrared detectors using narrow bandgap semiconductors (such as InSb and HgCdTe) with silicon charge coupled devices that performed data storage and image processing functions on arrays of pixels. A major research topic in my group in the charge-to-light category has for many years been electroluminescence [2] from wide bandgap semiconductors (primarily GaN and AlN) doped with rare earth elements (Er, Eu, Tm, etc.). Narrow linewidth emission can occur at wavelengths from near UV to visible to IR, depending on the electronic transitions of the selected RE element and the bandgap of the host material. In the past 10 years, my materials interest switched from inorganic to organic semiconductors and biomaterials, but with a continuing strong opto/electronic component. Currently, a major research focus is in developing simple, low cost and intuitive diagnostic devices for point-of-care use. These devices analyze the composition of biofluids (e.g. blood, sweat, saliva, etc.) to provide a 1st order indication of specific aspects the individual’s health status, such as blood coagulation, stress, etc. The use of light in PoC and other diagnostic devices is widespread. Most common and simplest approach is colorimetric detection in ambient light, where a change of color in a region of the device indicates the presence of a specific biomolecule associated with a given medical condition. Somewhat more complex diagnostic devices use fluorescence of molecules attached to biomarkers to indicate their presence. In our own research, biofluid samples are typically transported by capillary flow to a location where their key properties are measured by a combination of optical emission [3], transmission and absorption [4]. The presentation will conclude with a look forward to other applications where photonics and electronics could join forces.[1] A.J. Steckl, et al., “Application of Charge Coupled Devices to Infrared Detection and Imaging,” Proc. of IEEE, 63(1), 67, Jan 1975 ( doi.org/10.1109/PROC.1975.9708 ).[2] (a) A.J. Steckl and J.M. Zavada, “Optoelectronic Properties and Applications of Rare-Earth-Doped GaN”, Mat. Res. Soc. Bulletin, 24(9), 33, Sep 1999 doi.org/10.1557/S0883769400053045; (b) A. J. Steckl, et al., Invited Paper, “Rare-earth-doped GaN: Growth, Properties and Fabrication of Electroluminescent Devices”, IEEE J. Selected Topics in Quantum Electronics, 8 (4), 749 Jul 2002 doi.org/10.1109/JSTQE.2002.801690 [3] V. Venkatraman and A. J. Steckl, “Quantitative Detection in Lateral Flow Immunoassay Using Integrated Organic Optoelectronics”, IEEE Sensors, 17 (24), 8343, Dec 2017 (doi.org/10.1109/JSEN.2017.2764178 ).[4] P. Ray and A. J. Steckl, “Label-Free Optical Detection of Multiple Stress Biomarkers in Sweat, Plasma, Urine and Saliva”, ACS Sensors 4, 1346, 2019. (doi.org/10.1021/acssensors.9b00301). Figure 1

(Invited) Blood, Sweat and Tears... – Sensing of Human Performance Biomarkers in Bodily Fluids Using Point-of-Use Devices

Hormones produced by glands in the endocrine system and neurotransmitters produced by the nervous system control many bodily functions. The concentrations of these molecules in the body are an indication of its state, hence the use of the term biomarker. Excess concentrations of biomarkers, such as cortisol, serotonin, epinephrine, dopamine, are released by the body in response to a variety of conditions - emotional state (euphoria, stress), disease, etc. The development of simple, low-cost modalities for point-of-use (PoU) measurements of biomarkers levels in various bodily fluids (blood, urine, sweat, saliva) as opposed to conventional hospital or lab settings is receiving increasing attention. The presentation starts with a review of the basic properties of primary stress-related biomarkers: origin in the body (i.e. if they are produced as hormones, neurotransmitters or both), chemical composition, molecular weight (small/medium size molecules and polymers, ranging from ~100Da to ~100kDa), hydro- or lipo-philic nature. Next, a review of the published literature is presented regarding the concentration of these biomarkers found in several bodily fluids that can serve as the medium for determination of the condition of the subject: blood, urine, saliva, sweat and, to a lesser degree, interstitial tissue fluid. The concentration of various biomarkers in most fluids covers a range of 5-6 orders of magnitude, from 100s of ng/mL (~1µM) down to a few pg/mL (sub 1pM). Mechanisms and materials for point-of-use biomarker sensors are summarized and key properties are reviewed. Illustrative examples from the literature are discussed for several sensor device categories, including lateral flow (immuno)assay devices and microfluidic devices. Selected methods for detecting these biomarkers are reviewed, including antibody- and aptamer-based colorimetric assays, electrochemical and optical detection. Finally, the presentation outlines key challenges of the field and provides a look ahead to future prospects.A.J. Steckl and P. Ray, “A Review of Stress Biomarkers Concentrations in Bodily Fluids and Their Point-of-Use Detection”, ACS Sensors, 3 (10), 2025-2044, Sept 2018 (10.1021/acssensors.8b00726).V. Venkatraman and A.J. Steckl, “Quantitative Detection in Lateral Flow Immunoassay using Integrated Organic Optoelectronics”, IEEE Sensors, 17 (24), 8343, Dec 2017 (10.1109/JSEN.2017.2764178 ).P. Ray and A.J. Steckl, “Label-Free Optical Detection of Multiple Stress Biomarkers in Sweat, Plasma, Urine and Saliva”, ACS Sensors 4, 2019. (10.1021/acssensors.9b00301)S. Dalirirad and A.J. Steckl, “Aptamer-Based Lateral Flow Assay for Point of Care Cortisol Detection in Sweat”, Sensors & Actuators 283, 79-86, Jan 2019 (https://doi.org/10.1016/j.snb.2018.11.161). Figure 1

Label-free real time optical detection of binding of living cells and biopolymers

Biosensor based on long range surface waves on one-dimentional photonic crystal (PC) surface in microfluid channel was used to detect binding of cells, viruses, nanoparticles and proteins. Covalent attachment of biopolymers to polyaminated PC with glutaraldehyde cross-linking in microfluid channel results in functionally active proteins, living eukaryotic, bacterial cells and viruses. The evident advantages of the optical biosensor include native conformations of biopolymers, detection of reversible or permanent specific binding with ligands, broad detection scale from eukaryotic cells to low-molecular-weight ligands and observations in real time with possible calculations of affinity and kinetic constants. However, relatively low sensitivity limit near 0.1 μg/ml of proteins is significantly less those of ELISA and xMAP immunofluorescent analysis (approximately 1 pg/ml). Integral adlayer thickness does not reveal molecular events on each cell with possible variations. The label-free real-time optical detection allowed us to estimate cytotoxicity of currently used and novel drugs and to explore the antiviral properties of new compounds.

Label-free differentiation of human immunodeficiency virus-1 infected from uninfected cells using transmission measurement.

Transmission measurement has been perceived as a potential candidate for label-free investigation of biological material. It is a real-time, label-free and non-invasive optical detection technique that has found wide applications in pharmaceutical industry as well as the biological and medical fields. Combining transmission measurement with optical trapping has emerged as a powerful tool allowing stable sample trapping, while also facilitating transmittance data analysis. In this study, a near-infrared laser beam emitting at a wavelength of 1064 nm was used for both optical trapping and transmission measurement investigation of human immunodeficiency virus 1 (HIV-1) infected and uninfected TZM-bl cells. The measurements of the transmittance intensity of individual cells in solution were carried out using a home built optical trapping system combined with laser transmission setup using a single beam gradient trap. Transmittance spectral intensity patterns revealed significant differences between the HIV-1 infected and uninfected cells. This result suggests that the transmittance data analysis technique used in this study has the potential to differentiate between infected and uninfected TZM-bl cells without the use of labels. The results obtained in this study could pave a way into developing an HIV-1 label-free diagnostic tool with possible applications at the point of care .

Label-Free Optical Detection of Multiple Biomarkers in Sweat, Plasma, Urine, and Saliva.

We report a novel label-free quantitative detection of human performance "stress" biomarkers in different body fluids based on optical absorbance of the biomarkers in the ultraviolet (UV) region. Stress biomarker (hormones and neurotransmitters) concentrations in bodily fluids (blood, sweat, urine, saliva) predict the physical and mental state of the individual. The stress biomarkers primarily focused on in this manuscript are cortisol, serotonin, dopamine, norepinephrine, and neuropeptide Y. UV spectroscopy of stress biomarkers performed in the 190-400 nm range has revealed primary and secondary absorption peaks at near-UV wavelengths depending on their molecular structure. UV characterization of individual and multiple biomarkers is reported in various biofluids. A microfluidic/optoelectronic platform for biomarker detection is reported, with a prime focus toward cortisol evaluation. The current limit of detection of cortisol in sweat is ∼200 ng/mL (∼0.5 μM), which is in the normal (healthy) range. Plasma samples containing both serotonin and cortisol resulted in readily detectable absorption peaks at 203 (serotonin) and 247 (cortisol) nm, confirming feasibility of simultaneous detection of multiple biomarkers in biofluid samples. UV spectroscopy performed on various stress biomarkers shows a similar increasing absorption trend with concentration. The detection mechanism is label free, applicable to a variety of biomarker types, and able to detect multiple biomarkers simultaneously in various biofluids. A microfluidic flow cell has been fabricated on a polymer substrate to enable point-of-use/care UV measurement of target biomarkers. The overall sensor combines sample dispensing and fluid transport to the detection location with optical absorption measurements with a UV light emitting diode (LED) and photodiode. The biomarker concentration is indicated as a function of photocurrent generated at the target wavelength.

Optical Imaging of Charges with Atomically Thin Molybdenum Disulfide.

Mapping local surface charge distribution is critical to the understanding of various surface processes and also allows the detection of molecules binding to the surface. We show here that the optical absorption of monolayer MoS2 is highly sensitive to charge and demonstrate optical imaging of local surface charge distribution with this atomically thin material. We validate the imaging principle and perform charge sensitivity calibration with an electrochemical gate. We further show that binding of charged molecules to the atomically thin material leads to a large change in the image contrast, allowing determination of the charge of the adsorbed molecules. This capability opens possibilities for characterizing impurities and defects in two-dimensional materials and for label-free optical detection and charge analysis of molecules.

Surface Enhanced Raman Spectroscopy for Molecular Identification- a Review on Surface Plasmon Resonance (SPR) and Localised Surface Plasmon Resonance (LSPR) in Optical Nanobiosensing

Surface plasmon resonance (SPR) allows for real-time, label-free optical detection of many chemical and biological substances. Having emerged in the last two decades, it is a widely used technique due to its non-invasive nature, allowing for the ultra-sensitive detection of a number of analytes. This review article discusses the principles, providing examples and illustrating the utility of SPR within the frame of plasmonic nanobiosensing, while making comparisons with its successor, namely localized surface plasmon resonance (LSPR). In particular LSPR utilizes both metal nanoparticle arrays and single nanoparticles, as compared to a continuous film of gold as used in traditional SPR. LSPR, utilizes metal nanoparticle arrays or single nanoparticles that have smaller sizes than the wavelength of the incident light, measuring small changes in the wavelength of the absorbance position, rather than the angle as in SPR. We introduce LSPR nanobiosensing by describing the initial experiments performed, shift-enhancement methods, exploitation of the short electromagnetic field decay length, and single nanoparticle sensors are as pathways to further exploit the strengths of LSPR nanobiosensing. Coupling molecular identification to LSPR spectroscopy is also explored and thus examples from surface-enhanced Raman spectroscopy are provided. The unique characteristics of LSPR nanobiosensing are emphasized and the challenges using LSPR nanobiosensors for detection of biomolecules as a biomarker are discussed.

Label-Free Optical Detection of DNA Translocations through Plasmonic Nanopores.

Solid-state nanopores are single-molecule sensors that hold great potential for rapid protein and nucleic-acid analysis. Despite their many opportunities, the conventional ionic current detection scheme that is at the heart of the sensor suffers inherent limitations. This scheme intrinsically couples signal strength to the driving voltage, requires the use of high-concentration electrolytes, suffers from capacitive noise, and impairs high-density sensor integration. Here, we propose a fundamentally different detection scheme based on the enhanced light transmission through a plasmonic nanopore. We demonstrate that translocations of single DNA molecules can be optically detected, without the need of any labeling, in the transmitted light intensity through an inverted-bowtie plasmonic nanopore. Characterization and the cross-correlation of the optical signals with their electrical counterparts verify the plasmonic basis of the optical signal. We demonstrate DNA translocation event detection in a regime of driving voltages and buffer conditions where traditional ionic current sensing fails. This label-free optical detection scheme offers opportunities to probe native DNA–protein interactions at physiological conditions.

Label-free optical detection of aflatoxin by using a liquid crystal-based immunosensor

Aflatoxin is one of the most toxic mycotoxins and a severe threat to human health. Here, we report a novel liquid crystal (LC) cell system that can be used to visualize the immune competition reaction between an antibody and the target antigen aflatoxin. The sensing system involves co-immobilization of the antibody and N, N-dimethyl-N-octadecyl (3-aminopropyl) trimethoxysilyl chloride (DMOAP) on the surface of a glass slide, resulting in a random orientation of the LCs in the film on these surfaces. In this system, the LC molecules are disordered, producing bright optical signals upon binding of the antibody to the surface-immobilized antigen. In the presence of aflatoxin, the interaction between the target antigen and antibody results in an immune competition reaction, which disturbs the orientation of LCs. This change in orientation can be observed by the naked eye through a crossed polarizer. The results demonstrated that this method possesses high specificity with an antigen detection limit as low as 100 pg/mL. This method does not require laboratory-based instruments, and the test results can be observed with naked eyes through a polarized microscope.