Image Sensing of Gaseous Acetone Using Secondary Alcohol Dehydrogenase-Immobilized Mesh for Exhaled Air.

Human-borne acetone is a potent marker of lipid metabolism. Here, an enzyme immobilization method for secondary alcohol dehydrogenase (S-ADH), which is suitable for highly sensitive and selective biosensing of acetone, was developed, and then its applicability was demonstrated for spatiotemporal imaging of concentration distribution. After various investigations, S-ADH-immobilized meshes could be prepared with less than 5% variation by cross-linking S-ADH with glutaraldehyde on a cotton mesh at 40 °C for 15 min. Furthermore, high activity was obtained by adjusting the concentration of the coenzyme nicotinamide adenine dinucleotide (NADH) solution added to the S-ADH-immobilized mesh to 500 µM and the solvent to a potassium phosphate buffer solution at pH 6.5. The gas imaging system using the S-ADH-immobilized mesh was able to image the decrease in NADH fluorescence (ex 340 nm, fl 490 nm) caused by the catalytic reaction of S-ADH and the acetone distribution in the concentration range of 0.1-10 ppm-v, including the breath concentration of healthy people at rest. The exhaled breath of two healthy subjects at 6 h of fasting was quantified as 377 and 673 ppb-v, which were consistent with the values quantified by gas chromatography-mass spectrometry.

Mouthguard-Type Wearable Sensor for Monitoring Salivary Turbidity to Assess Oral Hygiene.

Salivary turbidity is a promising indicator for evaluating oral hygiene. This study proposed a wearable mouthguard-type sensor for continuous and unconstrained measurement of salivary turbidity. The sensor evaluated turbidity by measuring the light transmittance of saliva with an LED and a phototransistor sealed inside a double-layered mouthguard. The sensor was also embedded with a Bluetooth wireless module, enabling the wireless measurement of turbidity. The mouthguard materials (polyethylene terephthalate-glycol and ethylene-vinyl acetate) and the wavelength of the LED (405 nm) were experimentally determined to achieve high sensitivity in salivary turbidity measurement. The turbidity quantification characteristic of the proposed sensor was evaluated using a turbidity standard solution, and the sensor was capable of turbidity quantification over a wide dynamic range of 1-4000 FTU (formazine turbidity unit), including reported salivary turbidity (400-800 FTU). In vitro turbidity measurement using a saliva sample showed 553 FTU, which is equivalent to the same sample measured with a spectrophotometer (576 FTU). Moreover, in vivo experiments also showed results equivalent to that measured with a spectrophotometer, and wireless measurement of salivary turbidity was realized using the mouthguard-type sensor. Based on these results, the proposed mouthguard-type sensor has promising potential for the unconstrained continuous evaluation of oral hygiene.

Real-Time Continuous Monitoring of Oral Soft Tissue Pressure with a Wireless Mouthguard Device for Assessing Tongue Thrusting Habits.

In orthodontics, understanding the pressure of oral soft tissues on teeth is important to elucidate the cause and establish treatment methods. We developed a small wireless mouthguard (MG)-type device that continuously and unrestrainedly measures pressure, which had previously been unachieved, and evaluated its feasibility in human subjects. First, the optimal device components were considered. Next, the devices were compared with wired-type systems. Subsequently, the devices were fabricated for human testing to measure tongue pressure during swallowing. The highest sensitivity (51-510 g/cm2) with minimum error (CV < 5%) was obtained using an MG device with polyethylene terephthalate glycol and ethylene vinyl acetate for the lower and upper layers, respectively, and with a 4 mm PMMA plate. A high correlation coefficient (0.969) was observed between the wired and wireless devices. In the measurements of tongue pressure on teeth during swallowing, 132.14 ± 21.37 g/cm2 for normal and 201.17 ± 38.12 g/cm2 for simulated tongue thrust were found to be significantly different using a t-test (n = 50, p = 6.2 × 10-19), which is consistent with the results of a previous study. This device can contribute to assessing tongue thrusting habits. In the future, this device is expected to measure changes in the pressure exerted on teeth during daily life.

Gas-Phase Biosensors (Bio-Sniffers) for Measurement of 2-Nonenal, the Causative Volatile Molecule of Human Aging-Related Body Odor.

The molecule 2-nonenal is renowned as the origin of unpleasant human aging-related body odor that can potentially indicate age-related metabolic changes. Most 2-nonenal measurements rely on chromatographic analytical systems, which pose challenges in terms of daily usage and the ability to track changes in concentration over time. In this study, we have developed liquid- and gas-phase biosensors (bio-sniffers) with the aim of enabling facile and continuous measurement of trans-2-nonenal vapor. Initially, we compared two types of nicotinamide adenine dinucleotide (phosphate) [NAD(P)]-dependent enzymes that have the catalytic ability of trans-2-nonenal: aldehyde dehydrogenase (ALDH) and enone reductase 1 (ER1). The developed sensor quantified the trans-2-nonanal concentration by measuring fluorescence (excitation: 340 nm, emission: 490 nm) emitted from NAD(P)H that was generated or consumed by ALDH or ER1. The ALDH biosensor reacted to a variety of aldehydes including trans-2-nonenal, whereas the ER1 biosensor showed high selectivity. In contrast, the ALDH bio-sniffer showed quantitative characteristics for trans-2-nonenal vapor at a concentration range of 0.4-7.5 ppm (with a theoretical limit of detection (LOD) and limit of quantification (LOQ) of 0.23 and 0.26 ppm, respectively), including a reported concentration (0.85-4.35 ppm), whereas the ER1 bio-sniffer detected only 0.4 and 0.8 ppm. Based on these findings, headspace gas of skin-wiped alcohol-absorbed cotton collected from study participants in their 20s and 50s was measured by the ALDH bio-sniffer. Consequently, age-related differences in signals were observed, suggesting the potential for measuring trans-2-nonenal vapor.

Biochemical Methanol Gas Sensor (MeOH Bio-Sniffer) for Non-Invasive Assessment of Intestinal Flora from Breath Methanol.

Methanol (MeOH) in exhaled breath has potential for non-invasive assessment of intestinal flora. In this study, we have developed a biochemical gas sensor (bio-sniffer) for MeOH in the gas phase using fluorometry and a cascade reaction with two enzymes, alcohol oxidase (AOD) and formaldehyde dehydrogenase (FALDH). In the cascade reaction, oxidation of MeOH was initially catalyzed by AOD to produce formaldehyde, and then this formaldehyde was successively oxidized via FALDH catalysis together with reduction of oxidized form of ß-nicotinamide adenine dinucleotide (NAD+). As a result of the cascade reaction, reduced form of NAD (NADH) was produced, and MeOH vapor was measured by detecting autofluorescence of NADH. In the development of the MeOH bio-sniffer, three conditions were optimized: selecting a suitable FALDH for better discrimination of MeOH from ethanol in the cascade reaction; buffer pH that maximizes the cascade reaction; and materials and methods to prevent leaking of NAD+ solution from an AOD-FALDH membrane. The dynamic range of the constructed MeOH bio-sniffer was 0.32-20 ppm, which encompassed the MeOH concentration in exhaled breath of healthy people. The measurement of exhaled breath of a healthy subject showed a similar sensorgram to the standard MeOH vapor. These results suggest that the MeOH bio-sniffer exploiting the cascade reaction will become a powerful tool for the non-invasive intestinal flora testing.

A Wearable Cellulose Acetate-Coated Mouthguard Biosensor for In Vivo Salivary Glucose Measurement.

In this study, a cellulose acetate (CA) membrane is formed as an interference rejection membrane on a glucose sensor to measure glucose in saliva. Glucose in saliva is successfully measured in vivo without any pretreatment of human saliva. A mouthguard (MG) glucose sensor is developed to monitor salivary glucose, which is reported to be correlated with the blood glucose level. Salivary components of ascorbic acid (AA) and uric acid (UA) hinder the accurate measurement of the glucose concentration of human saliva. CA-coated electrodes are prepared to investigate the interference rejection membrane. To measure hydrogen peroxide, which is a reaction product of glucose oxidase, effects of AA and UA are examined. Characteristics of the fabricated biosensor are examined on the basis of artificial saliva. The as-developed MG sensor can quantify the glucose concentration in the range of 1.75-10 000 µmol/L, which includes a salivary sugar concentration of 20-200 µmol/L. For the measurement of saliva samples collected from healthy subjects, the output corresponding to the concentration is confirmed; this suggests the possibility of glucose measurement. This MG glucose sensor can provide a useful method for the unrestricted and noninvasive monitoring of saliva glucose for the management of diabetes patients.

Evaluation for regional difference of skin-gas ethanol and sweat rate using alcohol dehydrogenase-mediated fluorometric gas-imaging system (sniff-cam).

Skin gas that contains volatile metabolites (volatilome) is emanated continuously and is thus expected to be suitable for non-invasive monitoring. The aim of this study was to investigate the relationship between the regional difference of sweat rate and skin volatilome distribution to identify the suitable site to monitor metabolisms. In this study, we developed a biofluorometric gas-imaging system (sniff-cam) based on nicotinamide adenine dinucleotide (NAD)-dependent alcohol dehydrogenase (ADH) to visualize transcutaneous ethanol (EtOH) distribution. The EtOH distribution was converted to a fluorescence distribution of reduced NAD with autofluorescence property. First, we optimized the solution volume and concentration of the oxidized NAD, which was a coenzyme of ADH. Owing to the optimization, a two-dimensional distribution of EtOH could be visualized from 0.05-10 ppm with good sensitivity and selectivity. Subsequently, transcutaneous EtOH imaging and measurement of sweat rate were performed at the palm, dorsum of hand, and wrist of participants who consumed alcohol. Transcutaneous EtOH from all skin parts was imaged using the sniff-cam; the concentrations initially increased until 30 min after drinking, followed by a gradual decrease. Although the determined peak EtOH concentrations of typical subjects were approximately 1100 ± 35 ppb (palm), which were higher than 720 ± 18 ppb (dorsum) and 620 ± 13 ppb (wrist), the results of sweat rate suggested that the dorsum of hand and the wrist were appropriate sites. Finally, the sniff-cam could visualize the individual difference of alcohol metabolism capacity originating from aldehyde dehydrogenase phenotype by imaging transcutaneous EtOH.

Ultra-Sensitive Isopropanol Biochemical Gas Sensor (Bio-Sniffer) for Monitoring of Human Volatiles.

Our groups have previously developed a biochemical gas sensor to measure isopropanol (IPA) in exhaled air and have applied it for breath IPA investigation in healthy subjects and diabetes patients. In this study, the original bio-sniffer was modified with a series of components that improved the limit of detection (LOD). First, the modified IPA bio-sniffer used a C8855-type photomultiplier tube (PMT) that performed well in the photon sensitivity at the peak wavelength of nicotinamide adenine dinucleotide (NADH) fluorescence. Second, the multi-core bifurcated optical fiber, which incorporated 36 fibers to replace the previous dual-core type, enhanced the fluorescence collection. Third, the optical fiber probe was reinforced for greater width, and the flow-cell was redesigned to increase the area of the enzyme-immobilized membrane in contact with the air sample. These modifications lowered the detection limit to 0.5 ppb, a significant increase over the previous 1.0 ppb. Moreover, the modified bio-sniffer successfully analyzed the IPA concentration in exhaled air from a volunteer, which confirmed its capability for real-world sample detection. The modified bio-sniffer is more applicable to breath measurement and the detection of other extremely-low-concentration samples.

Ultrasensitive Sniff-Cam for Biofluorometric-Imaging of Breath Ethanol Caused by Metabolism of Intestinal Flora.

We developed a gas-imaging system (sniff-cam) for gaseous ethanol (EtOH) with improved sensitivity. The sniff-cam was applied to measure the extremely low concentration distribution of breath EtOH without the consumption of alcohol, which is related to the activity of the oral or gut bacterial flora. A ring-type ultraviolet-light-emitting diode was mounted around a camera lens as an excitation light source, which enabled simultaneous excitation and imaging of the fluorescence. In the EtOH sniff-cam, a nicotinamide adenine dinucleotide (NAD)-dependent alcohol dehydrogenase (ADH) was used to catalyze the redox reaction between EtOH and the oxidized form of NAD (NAD+). Upon application of gaseous EtOH to the ADH-immobilized mesh that was soaked in an NAD+ solution and placed in front of the camera, NADH was produced through an ADH-mediated reaction. NADH expresses fluorescence at an emission wavelength of 490 nm and excitation wavelength of 340 nm. Thus, the concentration distribution of EtOH was visualized by measuring the distribution of the fluorescence light intensity from NADH on the ADH-immobilized mesh surface. First, a comparison of image analysis methods based on the red-green-blue color (RGB) images and the optimization of the buffer pH and NAD+ solution concentration was performed. The new sniff-cam showed a 25-fold greater sensitivity and broader dynamic range (20.6-300000 ppb) in comparison to those of the previously fabricated sniff-cam. Finally, we measured the concentration distribution of breath EtOH without alcohol consumption using the improved sniff-cam and obtained a value of 116.2 ± 35.7 ppb (n = 10).

Fluorometric Sniff-Cam (Gas-Imaging System) Utilizing Alcohol Dehydrogenase for Imaging Concentration Distribution of Acetaldehyde in Breath and Transdermal Vapor after Drinking.

Understanding concentration distributions, release sites, and release dynamics of volatile organic compounds (VOCs) from the human is expected to lead to methods for noninvasive disease screening and assessment of metabolisms. In this study, we developed a visualization system (sniff-cam) that enabled one to identify a spatiotemporal change of gaseous acetaldehyde (AcH) in real-time. AcH sniff-cam was composed of a camera, a UV-LED array sheet, and an alcohol dehydrogenase (ADH)-immobilized mesh. A reverse reaction of ADH was employed for detection of gaseous AcH where a relationship between fluorescence intensity from nicotinamide adenine dinucleotide and the concentration of AcH was inversely proportional; thus, the concentration distribution of AcH was measured by detecting the fluorescence decrease. Moreover, the image differentiation method that calculated a fluorescence change rate was employed to visualize a real-time change in the concentration distribution of AcH. The dynamic range of the sniff-cam was 0.1-10 ppm which encompassed breath AcH concentrations after drinking. Finally, the sniff-cam achieved the visualization of the concentration distribution of AcH in breath and skin gas. A clear difference of breath AcH concentration was observed between aldehyde dehydrogenase type 2 active and inactive subjects, which was attributed to metabolic capacities of AcH. AcH in skin gas showed a similar time course of AcH concentration to the breath and a variety of release concentration distribution. Using different NADH-dependent dehydrogenases in the sniff-cam could lead to a versatile method for noninvasive disease screening by acquiring spatiotemporal information on various VOCs in breath or skin gas.

Fluorometric Biosniffer Camera "Sniff-Cam" for Direct Imaging of Gaseous Ethanol in Breath and Transdermal Vapor.

Various volatile organic compounds can be found in human transpiration, breath and body odor. In this paper, a novel two-dimensional fluorometric imaging system, known as a "sniffer-cam" for ethanol vapor released from human breath and palm skin was constructed and validated. This imaging system measures ethanol vapor concentrations as intensities of fluorescence through an enzymatic reaction induced by alcohol dehydrogenase (ADH). The imaging system consisted of multiple ultraviolet light emitting diode (UV-LED) excitation sheet, an ADH enzyme immobilized mesh substrate and a high-sensitive CCD camera. This imaging system uses ADH for recognition of ethanol vapor. It measures ethanol vapor by measuring fluorescence of nicotinamide adenine dinucleotide (NADH), which is produced by an enzymatic reaction on the mesh. This NADH fluorometric imaging system achieved the two-dimensional real-time imaging of ethanol vapor distribution (0.5-200 ppm). The system showed rapid and accurate responses and a visible measurement, which could lead to an analysis of metabolism function at real time in the near future.

Biochemical Gas Sensors (Biosniffers) Using Forward and Reverse Reactions of Secondary Alcohol Dehydrogenase for Breath Isopropanol and Acetone as Potential Volatile Biomarkers of Diabetes Mellitus.

This study describes two biosniffers to determine breath acetone and isopropanol (IPA) levels and applies them for breath measurement in healthy subjects and diabetic patients. Secondary alcohol dehydrogenase (S-ADH) can reduce acetone and oxidize nicotinamide adenine dinucleotide (NADH to NAD+) in a weak acid environment. NADH can be excited by 340 nm excitation lights and subsequently emit 490 nm fluorescence. Therefore, acetone can be measured by the decrease in NADH fluorescence intensity. S-ADH can also oxidize IPA and reduce NAD+ to NADH when it is in an alkaline environment. Thus, IPA can be detected by the increase of fluorescence. The developed biosniffers show rapid response, high sensitivity and high selectivity. The breath acetone and IPA analysis in healthy subjects shows that the mean values were 750.0 ± 434.4 ppb and 15.4 ± 11.3 ppb. Both acetone and IPA did not show a statistical difference among different genders and ages. The breath acetone analysis for diabetic patients shows a mean value of 1207.7 ± 689.5 ppb, which was higher than that of healthy subjects (p < 1 × 10-6). In particularly, type-1 diabetic (T1D) patients exhaled a much higher concentration of acetone than type-2 diabetic (T2D) patients (p < 0.01). The breath IPA also had a higher concentration in diabetic patients (23.1 ± 20.1 ppb, p < 0.01), but only T2D patients presented a statistical difference (23.9 ± 21.3 ppb, p < 0.01). These findings are worthwhile in the study of breath biomarkers for diabetes mellitus diagnosis. Additionally, the developed biosniffers provide a new technique for volatolomics research.

Fluorometric gas-imaging system (sniff-cam), using the extinction of NADH with an ADH reverse reaction, for acetaldehyde in the gas phase.

A gas-imaging system (sniff-cam) that allows fluorometric visualization of a two-dimensional (2-D) distribution of gaseous acetaldehyde (AcH) was developed. It employed a reverse reaction of a nicotinamide adenine dinucleotide (NADH) dependent enzyme that led to consumption of NADH in that reaction. The system was constructed with a highly sensitive camera, an ultraviolet light emitting diode array sheet, two band pass filters and an alcohol dehydrogenase (ADH)-immobilized mesh that was used for AcH detection. The reverse reaction of the ADH catalyzed the reduction of AcH to ethanol and the oxidation of NADH to NAD+, which occurred when gaseous AcH was applied to the ADH immobilized mesh that was wetted with a slightly acidic NADH solution. As NADH has an autofluorescence property [emission (λem) at 490 nm; excitation (λex) at 340 nm], the presence of gaseous AcH was visualized by a decrease of fluorescence of the NADH at the ADH immobilized mesh. After constructing the gaseous AcH imaging system, optimizations of pH, and concentration of the NADH solution were performed. As a result of the optimizations (500 µM of NADH in 0.1 M of Tris hydrochloride (Tris-HCl) buffer at pH 6.5), the AcH sniff-cam showed a wide dynamic range (0.1-10 ppm) for gaseous AcH with a high correlation coefficient (R = 0.999). Furthermore, a fluorescence gradient with a rounded shape centered in a gas outlet was observed. These results demonstrated that the AcH sniff-cam utilizing the fluorescence decrease of NADH could be used to quantitatively evaluate the 2-D distribution of gaseous AcH.

Repetitive Immunoassay with a Surface Acoustic Wave Device and a Highly Stable Protein Monolayer for On-Site Monitoring of Airborne Dust Mite Allergens.

This work describes a sensor to be incorporated into the on-site monitoring system of airborne house dust mite (HDM) allergens. A surface acoustic wave (SAW) device was combined with self-assembled monolayers of a highly stable antibody capture protein on the SAW surface that have high resistance to pH change. A sandwich assay was used to measure a HDM allergen, Der f 1 derived from Dermatophagoides farinae. Capture antibodies were cross-linked to a protein G based capture layer (ORLA85) on the sensor surface, thereby only Der f 1 and detection antibodies were regenerated by changing pH, resulting in fast repetition of the measurement. The sensor was characterized through 10 repetitive measurements of Der f 1, which demonstrated high reproducibility of the sensor with the coefficient of variation of 5.6%. The limit of detection (LOD) of the sensor was 6.1 ng·mL(-1), encompassing the standard (20 ng·mL(-1)) set by the World Health Organization. Negligible sensor outputs were observed for five different major allergens including other HDM allergens which tend to have cross-reactivity to Der f 1 and their mixtures with Der f 1. Finally, the sensor lifetime was evaluated by conducting three measurements per day, and the sensor output did not substantially change for 4 days. These characteristics make the SAW immunosensor a promising candidate for incorporation into on-site allergen monitoring systems.

A sniffer-camera for imaging of ethanol vaporization from wine: the effect of wine glass shape.

A two-dimensional imaging system (Sniffer-camera) for visualizing the concentration distribution of ethanol vapor emitting from wine in a wine glass has been developed. This system provides image information of ethanol vapor concentration using chemiluminescence (CL) from an enzyme-immobilized mesh. This system measures ethanol vapor concentration as CL intensities from luminol reactions induced by alcohol oxidase and a horseradish peroxidase (HRP)-luminol-hydrogen peroxide system. Conversion of ethanol distribution and concentration to two-dimensional CL was conducted using an enzyme-immobilized mesh containing an alcohol oxidase, horseradish peroxidase, and luminol solution. The temporal changes in CL were detected using an electron multiplier (EM)-CCD camera and analyzed. We selected three types of glasses-a wine glass, a cocktail glass, and a straight glass-to determine the differences in ethanol emission caused by the shape effects of the glass. The emission measurements of ethanol vapor from wine in each glass were successfully visualized, with pixel intensity reflecting ethanol concentration. Of note, a characteristic ring shape attributed to high alcohol concentration appeared near the rim of the wine glass containing 13 °C wine. Thus, the alcohol concentration in the center of the wine glass was comparatively lower. The Sniffer-camera was demonstrated to be sufficiently useful for non-destructive ethanol measurement for the assessment of food characteristics.

Biofluorometric Gas-Imaging System for Evaluating the Ripening Stages of "La France" Pear Based on Ethanol Vapor Emitted via the Epicarp.

Fruits can emit ethanol, which is generated through fermentation during hypoxic storage. We imaged spatiotemporal changes in the gaseous ethanol emitted by "La France" pear via its epicarp. The gas-imaging system utilized enzymes to transduce the ethanol concentration into fluorescence intensity. Initially, the uniformity of the enzyme and coenzyme distribution was evaluated to validate the imaging capability. Subsequently, two surface-fitting methods were compared to accurately image ethanol emitted from three-dimensional (3D) objects with a double-curved surface. The imaging results of ethanol emitted from the pear indicated that the distribution of ethanol was related to lenticels, which have been reported to possess high ethanol diffusivity, on the epicarp. As quantified by the system (uniformity of coenzyme and enzymes was 93.2 and 98.8%, respectively; dynamic range was 0.01-100 ppm), ethanol concentration increased with the storage period under hypoxic conditions (0.4-5.3 ppm, from day 1 to 10). The system enables the observation of the location, quantity, and temporal pattern of ethanol release from fruit, which could be a useful technology for agricultural applications.

In Vitro Performance of a Long-Range Surface Plasmon Hydrogel Aptasensor for Continuous and Real-Time Vancomycin Measurement in Human Serum.

This study investigated the suitability of surface modification for a long-range surface plasmon (LRSP) aptasensor using two different hydrogels, aiming at real-time monitoring of vancomycin (VCM) in undiluted serum and blood. Three different layer structures were formed on a gold surface of LRSP sensor chip using poly[2-methacryloyloxyethyl phosphorylcholine (MPC)-co-N-methacryloyl-(L)-tyrosinemethylester (MAT)] (PMM) and poly[MPC-co-2-ethylhexyl methacrylate (EHMA)-co-MAT] (PMEM). The peptide aptamer for VCM was immobilized in PMM and PMEM via MAT. Among four differently prepared sensor chips, the LRSP hydrogel aptasensor with PMM, referred to as the PMM hydrogel, exhibited the highest sensor output and superior antifouling properties. Following the optimization of the PMM hydrogel preparation conditions, the shelf life of the PMM hydrogel was determined to exceed 2 weeks, and the same sensor chip could be used for 102 days without significant performance deterioration. The PMM hydrogel was then applied for VCM measurement in undiluted serum in vitro, where it demonstrated a limit of detection of 0.098 µM and a dynamic range of 0.18-100 µM, covering the therapeutic range. Additionally, the PMM hydrogel enabled the continuous measurement of various VCM concentrations in serum without rinsing and showed a concentration-dependent output in undiluted blood. These findings underscore the potential of the PMM hydrogel for real-time and direct monitoring of VCM in body fluids.

Compact surface plasmon-enhanced fluorescence biochip.

A new concept of compact biochip for surface plasmon-enhanced fluorescence assays is reported. It takes advantage of the amplification of fluorescence signal through the coupling of fluorophore labels with confined and strongly enhanced field intensity of surface plasmons. In order to efficiently excite and collect the emitted fluorescence light via surface plasmons on a metallic sensor surface, (reverse) Kretschmann configuration is combined with diffractive optical elements embedded on the chip surface. These include a concentric relief grating for the imaging of highly directional surface plasmon-coupled emission to a detector. Additional linear grating is used for the generating of surface plasmons at the excitation wavelength on the sensor surface in order to increase the fluorescence excitation rate. The reported approach offers the increased intensity of fluorescence signal, reduced background, and compatibility with nanoimprint lithography for cost-effective preparation of sensor chip. The presented approach was implemented for biosensing in a model immunoassay experiment in which the limit of detection of 11 pM was achieved.

Long-range surface plasmon aptasensor for label-free monitoring of vancomycin.

Vancomycin (VCM) causes poisoning symptoms at high concentrations; thus, therapeutic drug monitoring is recommended to measure and control blood levels regularly. However, blood analysis at regular intervals does not allow knowing the detailed temporal change in concentration. To address this challenge, we developed a long-range surface plasmon (LRSP) aptasensor for measuring VCM label-free and real-time by combining a sensitive LRSP sensor and a peptide aptamer with a VCM recognition site. First, three different biosensors for VCM were compared. One was prepared by immobilizing the peptide aptamer directly on (Direct-Apt) or via a self-assembled monolayer (SAM) on a gold surface (SAM-Apt). The other used anti-VCM antibodies immobilized on a gold surface via the SAM (SAM-Ab). The Direct-Apt showed larger sensor output to VCM than the other biosensors. The dynamic range for VCM was 0.78-100 µM, including the therapeutic range (6.9-13.8 µM). The Direct-Apt also showed the sensor output only from VCM among four different antibiotics, demonstrating the high selectivity for VCM. The VCM captured by the aptamer could be removed by rinsing with phosphate-buffered saline. The measurement was rapid, with 72- and 77-sec response and recovery times, allowing not only repeated but also real-time measurements. Finally, the Direct-Apt in 20% serum solutions showed comparable sensitivity to VCM in the buffer solution, indicating high capability for real-sample.

Surface plasmon-coupled emission on plasmonic Bragg gratings.

Surface plasmon-coupled emission (SPCE) from emitters in a close proximity to a plasmonic Bragg grating is investigated. In this study, the directional fluorescence emission mediated by Bragg-scattered surface plasmons and surface plasmons diffraction cross-coupled through a thin metallic film is observed by using the reverse Kretschmann configuration. We show that controlling of dispersion relation of these surface plasmon modes by tuning the refractive index at upper and lower interfaces of a dense sub-wavelength metallic grating enables selective reducing or increasing the intensity of the light emitted to certain directions. These observations may provide important leads for design of advanced plasmonic structures in applications areas of plasmon-enhanced fluorescence spectroscopy and nanoscale optical sources.