Amorphous Silicon Photosensors Research Articles

Monolithically integrated optical interference and absorption filters on thin film amorphous silicon photosensors for biological detection

High selectivity of photosensors is needed for targeted biological detection. Optical interference filters are monolithically integrated with hydrogenated amorphous silicon thin film photosensors for fluorescence measurements in lab-on-chip applications using a methodology that enables customization of fluorescence detection for various applications. The design of the optical interference filters is tailored to match the requirements of each application and the filters are fabricated as nitride-oxide multilayers. The interference filters are combined with hydrogenated amorphous silicon-carbon alloys, which serve as long-pass absorption filters, depending on the spectral requirements. In this work we demonstrate the development of these filters, through simulations and design, followed by experimental fabrication and characterization of transmission spectra, and finally, integration with photosensors which are implemented in measurements to detect fluorescence in three different applications, including bacteriophage fluorescence, quantum dot and Cy3 fluorophores, and grape fluorescence for grape maturation monitoring, in a novel methodology that allows for simplification and greater portability of the measurement setup and negates the need to stack optical filters in fluorescence applications.

Split Aptamers Immobilized on Polymer Brushes Integrated in a Lab-on-Chip System Based on an Array of Amorphous Silicon Photosensors: A Novel Sensor Assay.

Innovative materials for the integration of aptamers in Lab-on-Chip systems are important for the development of miniaturized portable devices in the field of health-care and diagnostics. Herein we highlight a general method to tailor an aptamer sequence in two subunits that are randomly immobilized into a layer of polymer brushes grown on the internal surface of microfluidic channels, optically aligned with an array of amorphous silicon photosensors for the detection of fluorescence. Our approach relies on the use of split aptamer sequences maintaining their binding affinity to the target molecule. After binding the target molecule, the fragments, separately immobilized to the brush layer, form an assembled structure that in presence of a “light switching” complex [Ru(phen)2(dppz)]2+, emit a fluorescent signal detected by the photosensors positioned underneath. The fluorescent intensity is proportional to the concentration of the target molecule. As proof of principle, we selected fragments derived from an aptamer sequence with binding affinity towards ATP. Using this assay, a limit of detection down to 0.9 µM ATP has been achieved. The sensitivity is compared with an assay where the original aptamer sequence is used. The possibility to re-use both the aptamer assays for several times is demonstrated.

On-Glass Integrated SU-8 Waveguide and Amorphous Silicon Photosensor for On-Chip Detection of Biomolecules: Feasibility Study on Hemoglobin Sensing

An optoelectronic, integrated system-on-glass for on-chip detection of biomolecules is here presented. The system’s working principle is based on the interaction, detected by a hydrogenated amorphous silicon photosensor, between a monochromatic light travelling in a SU-8 polymer optical waveguide and the biological solution under analysis. Optical simulations of the waveguide coupling to the thin-film photodiode with a specific design were carried out. A prototype was fabricated and characterized showing waveguide optical losses of about 0.6 dB/cm, a photodiode shot noise current of about 2.5 fA/ and responsivity of 495 mA/W at 532 nm. An electro-optical coupling test was performed on the fabricated device to validate the system. As proof of concept, hemoglobin was studied as analyte for a demonstration scenario, involving optical simulations interpolated with experimental data. The calculated detection limit of the proposed system for hemoglobin concentration in aqueous solution is around 100 ppm, in line with colorimetric methods currently on the market. These results show the effectiveness of the proposed system in biological detection applications and encourage further developments in implementing these kinds of devices in the biomedical field.

Fluorescent Label-Free Aptasensor Integrated in a Lab-on-Chip System for the Detection of Ochratoxin A in Beer and Wheat.

This paper reports on the development of a fluorescent label-free aptamer assay integrated in a lab-on-chip (LoC) system for the detection of Ochratoxin A (OTA). The detection system relies on the integration, on a single glass substrate, of an array of amorphous silicon photosensors and a long pass interferential filter. The aptamer assay, integrated into the microfluidic network, is an aptasensor having affinity versus OTA, selected as a case study. The fluorescent molecule is a "light switch" complex [Ru(phen)2(dppz)]2+. The aptamer is directly anchored into a layer of poly(2-hydroxyethyl methacrylate) polymer brushes grown inside the channels. The fluorophore is intercalated between the base pairs of the aptamer. Upon the interaction of OTA with the aptasensor, a change of the aptamer conformation causes the release of the fluorophore, yielding a decrease of the fluorescent signal detected by the array of the amorphous silicon photosensors positioned underneath the microfluidic network. The developed LoC is a portable system capable of performing the analysis with a small volume of sample (about 10 μL) in a short time (5 min) with a limit of detection for OTA equal to 1.3 ng/mL. The LoC has been applied for the detection of OTA (5-200 ng/mL)in beer and wheat samples.

On-chip real-time monitoring of multiple displacement amplification of DNA

The integration of systems for real-time monitoring of DNA amplification reactions is rather limited, because the heating system, the temperature control and the detection system are usually realized with separate modules. In this work, a lab-on-a-chip system for real-time monitoring of the multiple displacement amplification reaction (MDA) is presented. The amplification and detection unit consists of a system-on-glass (SoG) coupled to a microfluidic chip. The SoG includes thin film metallic resistors and amorphous silicon temperature sensors to control the temperature, as well as amorphous silicon photosensors and an interference filter for fluorescence detection on a single glass plate. The amplification reaction is carried out in the microfluidic chip made of cyclic olefin copolymer (COC). By using this setup, the multiple displacement amplification reaction can be monitored in real-time using the fluorophore [Ru(phen)2(dppz)]2+.

Integrated Optoelectronic Device for Detection of Fluorescent Molecules.

This paper presents the development of a compact optoelectronic device suitable for on-chip detection of fluorescent molecules. In order to obtain a highly integrated device, a long-pass multi-dielectric filter has been integrated with thin-film amorphous silicon photosensors on a single glass substrate. Filter rejects the excitation light, allowing the reduction of the distance between the source and the fluorescent site and avoiding the use of external optical component. The compatibility of the technological processes determined the materials and the temporal sequence of the device fabrication. The developed device has been designed for the fluorescence detection of ruthenium complex based molecules and tested, as a proof of concept, for the detection of double-stranded DNA down to 0.5 ng. Results demonstrate the correct operation of the integrated system in both rejecting the excitation light and in detecting the fluorescent signal, demonstrating the suitability of this optoelectronic platform in practical biomedical applications.

Integrated chemiluminescence-based lab-on-chip for detection of life markers in extraterrestrial environments

The detection of life markers is a high priority task in the exploration of the Solar System. Biochips performing in-situ multiplex immunoassays are a very promising approach alternative to gas chromatography coupled with mass spectrometry. As part of the PLEIADES project, we present the development of a chemiluminescence-based, highly integrated analytical platform for the detection of biomarkers outside of the Earth. The PLEIADES device goes beyond the current lab-on-chip approaches that still require bulky external instrumentation for their operation. It exploits an autonomous capillary force-driven microfluidic network, an array of thin-film hydrogenated amorphous silicon photosensors, and chemiluminescence bioassays to provide highly sensitive analyte detection in a very simple and compact configuration. Adenosine triphosphate was selected as the target life marker. Three bioassay formats have been developed, namely (a) a bioluminescence assay exploiting a luciferase mutant with enhanced thermal and pH stability and (b and c) binding assays exploiting antibodies or functional nucleic acids (aptamers) as biospecific recognition elements and peroxidase or DNAzymes as chemiluminescence reporters. Preliminary results, showing limits of detection in the nanomolar range, confirm the validity of the proposed approach.

Optoelectronic System for Mycotoxin Detection in Food Quality Control

Here, we report on the design, fabrication, and characterization of a prototype of a portable detection system, able to determine if the contamination level of Ochratoxin A (OTA) is below or above the limit imposed by the EU Commission for a specific food. The operating principle of the system is the real-time monitoring of a chromatographic run when an extract of the sample under analysis is spotted on a high-performance thin-layer chromatographic plate. The idea is to induce naturally the fluorescence of the OTA molecules by using an ultraviolet excitation source and to detect the re-emitted light with an array of amorphous silicon photosensors, whose photocurrents will be proportional to the amount of OTA molecules. The system performances have been verified on sample containing pure OTA molecule and OTA extracted from fortified real matrices of wine and beer. The results showed a minimum detectable quantity of OTA of 0.2 ng and repeatability better than 5%. The analysis of the sample is made simultaneously with a reference, which, in the proposed system, corresponds to the amount of OTA equal to the limit for the specific food commodities. Taking into account the extraction efficiency and the minimum acceptable presence of OTA in such food commodities, we infer that the minimum amount of real matrix to be investigated is as low as 5.5 mL.

An All-Glass Microfluidic Network with Integrated Amorphous Silicon Photosensors for on-Chip Monitoring of Enzymatic Biochemical Assay

A lab-on-chip system, integrating an all-glass microfluidics and on-chip optical detection, was developed and tested. The microfluidic network is etched in a glass substrate, which is then sealed with a glass cover by direct bonding. Thin film amorphous silicon photosensors have been fabricated on the sealed microfluidic substrate preventing the contamination of the micro-channels. The microfluidic network is then made accessible by opening inlets and outlets just prior to the use, ensuring the sterility of the device. The entire fabrication process relies on conventional photolithographic microfabrication techniques and is suitable for low-cost mass production of the device. The lab-on-chip system has been tested by implementing a chemiluminescent biochemical reaction. The inner channel walls of the microfluidic network are chemically functionalized with a layer of polymer brushes and horseradish peroxidase is immobilized into the coated channel. The results demonstrate the successful on-chip detection of hydrogen peroxide down to 18 μM by using luminol and 4-iodophenol as enhancer agent.

Performance of hydrogenated amorphous silicon thin film photosensors at ultra-low light levels: towards attomole sensitivities in labon- chip biosensing applications

Thin-film optical transducers integrated in lab-on-a-chip (LoC) devices have the potential to facilitate miniaturization, multiplexing capabilities, portability, and sensitivity. However, there are few systematic studies that provide detailed characterization of these photosensors to particular miniaturized bioassays at reduced light intensities. A more detailed representation of the photosensors performance at low light levels is thus needed in order to improve the engineering of integrated detectors for the next generation of portable biosensors. Here, an assessment of the performance of hydrogenated amorphous silicon (a-Si:H) photosensors, at ultra-low light intensity, based on two different device configurations for integration in a microfluidic platform for biomolecular detection is presented. Both p-i-n photodiodes and parallel contact photoconductors show low dark current density (~10−10 A.cm−2) and photosensitivity comparable with high performing crystalline silicon photosensors. The a-Si:H photosensors were integrated in microfluidic devices for the detection of antibody-horseradish peroxidase conjugates (Ab-HRP) in a chemiluminescence-based assay. Surface concentrations of Ab-HRP as low as ~26 amol.cm−2 and ~9 fmol.cm−2 (corresponding to ~103 and ~106 HRP molecules) were detected with detectors in photodiode and photoconductor configurations, respectively, with the sensitivity of the former being comparable with that of bulky and ultra-sensitive photomultiplier tubes. The excellent sensitivity and dynamic range obtained, together with the miniaturization and arraying potential, highlight the potential of a-Si:H photosensors as an effective means of integrating the multiplexed acquisition of optical data in real time in point-of-need LoC systems.

Chemiluminescence lateral flow immunoassay cartridge with integrated amorphous silicon photosensors array for human serum albumin detection in urine samples.

A novel and disposable cartridge for chemiluminescent (CL)-lateral flow immunoassay (LFIA) with integrated amorphous silicon (a-Si:H) photosensors array was developed and applied to quantitatively detect human serum albumin (HSA) in urine samples. The presented analytical method is based on an indirect competitive immunoassay using horseradish peroxidase (HRP) as a tracer, which is detected by adding the luminol/enhancer/hydrogen peroxide CL cocktail. The system comprises an array of a-Si:H photosensors deposited on a glass substrate, on which a PDMS cartridge that houses the LFIA strip and the reagents necessary for the CL immunoassay was optically coupled to obtain an integrated analytical device controlled by a portable read-out electronics. The method is simple and fast with a detection limit of 2.5mgL-1 for HSA in urine and a dynamic range up to 850mgL-1, which is suitable for measuring physiological levels of HSA in urine samples and their variation in different diseases (micro- and macroalbuminuria). The use of CL detection allowed accurate and objective analyte quantification in a dynamic range that extends from femtomoles to picomoles. The analytical performances of this integrated device were found to be comparable with those obtained using a charge-coupled device (CCD) as a reference off-chip detector. These results demonstrate that integrating the a-Si:H photosensors array with CL-LFIA technique provides compact, sensitive and low-cost systems for CL-based bioassays with a wide range of applications for in-field and point-of-care bioanalyses. Graphical Abstract A novel integrated portable device was developed for direct quantitative detection of human serum albumin (HSA) in urine samples, exploiting a chemiluminescence lateral flow immunoassay (LFIA). The device comprises a cartridge that holds the LFIA strip and all the reagents necessary for the analysis, an array of amorphous silicon photosensors, and a custom read-out electronics.

A point-of-use microfluidic device with integrated photodetector array for immunoassay multiplexing: Detection of a panel of mycotoxins in multiple samples

For a point-of-use analytical device to be successful in real-world applications, it needs to be rapid, simple to operate and, ideally, able to multiplex the detection of several analytes and samples.Mycotoxin detection in food and feedstock in particular has become increasingly relevant as these toxins, such as ochratoxin A (OTA), aflatoxin B1 (AFB1) and deoxynivalenol (DON), are subject to strict regulations and recommendations in the European Union. A novel, simple, negative pressure-driven device with manually operated magnetic valves was developed and the simultaneous immunodetection of these three mycotoxins was demonstrated via the laminar flow patterning of probes in an area of ≈0.12mm2 and subsequent chemiluminescence generation via HRP-labeled antibodies. The three mycotoxins were detected in less than 20min at concentrations of 100ng/mL for OTA and DON and 3ng/mL for AFB1, spiked in a sample under analysis and simultaneously compared to a toxin-free reference and a standard contaminated with critical target concentrations. The on-chip optical detection was performed in a single acquisition step by integrating a microfabricated array of 25×25µm2 hydrogenated amorphous silicon (a-Si:H) photosensors below the microfluidic chip. The device presented in this work is simple and effective for point-of-use multiplexing of immunoassays and was applied in this work to the screening of mycotoxins.

Aptamer-based sandwich assay for on chip detection of Ochratoxin A by an array of amorphous silicon photosensors

A multichannel glass-PDMS microfluidic chip functionalized with a novel aptasensor has been developed for the detection of Ochratoxin A (OTA). The aptasensor is built on a glass substrate covered with a poly(2-hydroxyethtyl methacrylate) brush layer, which is subsequently functionalized with an aptamer having high affinity towards OTA. The assay relies on an aptamer-linked immobilized sorbent assay (ALISA) using two different OTA aptamers, which, in presence of OTA, assemble to form a sandwich-like structure generating a chemiluminescent signal. The chip is combined with an array of amorphous silicon photosensors that transduce the chemiluminescent signal into an electrical signal that can be processed to detect and quantify the OTA in the sample. The successful detection of OTA has been demonstrated in standard solutions and in beer samples spiked with OTA. The results shows that the values measured for OTA, applying the ALISA assay, are comparable to those measured by the reference methods. The sensitivity of the proposed analytical method is 0.32pAL/mg, while the LOD and the LOQ are 0.82 and 2.5mg/L, respectively. Taking into account both the LOD of the ALISA and the method applied for OTA extraction from beer samples, we extrapolated that 1.1g of beer sample is sufficient to detect OTA contamination under the regulatory limits.

Microfluidic cartridge with integrated array of amorphous silicon photosensors for chemiluminescence detection of viral DNA

Portable and simple analytical devices based on microfluidics with chemiluminescence detection are particularly attractive for point-of-care applications, offering high detectability and specificity in a simple and miniaturized analytical format. Particularly relevant for infectious disease diagnosis is the ability to sensitively and specifically detect target nucleic acid sequences in biological fluids. To reach the goal of real-life applications for such devices, however, several technological challenges related to full device integration are still to be solved, one key aspect regarding on-chip integration of the chemiluminescence signal detection device. Nowadays, the most promising approach is on-chip integration of thin-film photosensors.We recently proposed a portable cartridge with microwells aligned with an array of hydrogenated amorphous silicon (a-Si:H) photosensors, reaching attomole level limits of detection for different chemiluminescence model reactions. Herein, we explore its applicability and performance for multiplex and quantitative detection of viral DNA. In particular, the cartridge was modified to accommodate microfluidic channels and, upon immobilization of three oligonucleotide probes in different positions along each channel, each specific for a genotype of Parvovirus B19, viral nucleic acid sequences were captured and detected.With this system, taking advantage of oligoprobes specificity, chemiluminescence detectability, and photosensor sensitivity, accurate quantification of target analytes down to 70pmolL−1 was obtained for each B19 DNA genotype, with high specificity and multiplexing ability. Results confirm the good detection capabilities and assay applicability of the proposed system, prompting the development of innovative portable analytical devices with enhanced sensitivity and multiplexed capabilities.

Lab-on-chip system combining a microfluidic-ELISA with an array of amorphous silicon photosensors for the detection of celiac disease epitopes

This work presents a lab-on-chip system, which combines a glass-polydimethilsiloxane microfluidic network and an array of amorphous silicon photosensors for the diagnosis and follow-up of Celiac disease. The microfluidic chip implements an on-chip enzyme-linked immunosorbent assay (ELISA), relying on a sandwich immunoassay between antibodies against gliadin peptides (GPs) and a secondary antibody marked with horseradish peroxidase (Ig-HRP). This enzyme catalyzes a chemiluminescent reaction, whose light intensity is detected by the amorphous silicon photosensors and transduced into an electrical signal that can be processed to recognize the presence of antibodies against GPs in the serum of people affected by Celiac syndrome.The correct operation of the developed lab-on-chip has been demonstrated using rabbit serum in the microfluidic ELISA. In particular, optimizing the dilution factors of both sera and Ig-HRP samples in the flowing solutions, the specific and non-specific antibodies against GPs can be successfully distinguished, showing the suitability of the presented device to effectively screen celiac disease epitopes.

Amorphous silicon photosensors integrated in microfluidic structures as a technological demonstrator of a “true” Lab-on-Chip system

In this paper we present a compact technological demonstrator including on the same glass substrate an electrowetting-on-dielectrics (EWOD) system, a linear array of amorphous silicon photosensor and a capillary-driven microfluidic channel. The proposed system comprises also a compact modular electronics controlling the digital microfluidics through the USB interface of a computer. The system provides therefore both on-chip detection and microfluidic handling needed for the realization of a ‘true’ Lab-on-Chip.The geometry of the photosensors has been designed to maximize the radiation impinging on the photosensor and to minimize the inter-site crosstalk, while the fabrication process has been optimized taking into account the compatibility of all the technological steps for the fabrication of the EWOD system, the photosensor array and the microfluidics channels.As a proof of the successful integration of the different technological steps we demonstrated the ability of the a-Si:H photosensors to detect the presence of a droplet over an EWOD electrode and the effective coupling between the digital and the continuous microfluidics, that can allow for functionalization, immobilization and recognition of biomolecules without external optical devices or microfluidic interconnections.

Amorphous silicon photosensors for on-chip detection in digital microfluidic system

In this paper we present the integration, on a single glass substrate, of amorphous silicon photodiodes with an electrowetting-on-dielectric (EWOD) system as a technological demonstrator for achieving a compact, stand alone Lab-on-Chip (LoC) system. The EWOD system comprises a thin film of indium tin oxide (ITO) acting as actuation electrodes and a 1μm-thick polydimethylsiloxane (PDMS) layer acting as both insulation and hydrophobic layer. The a-Si:H photosensors are ITO/p-type/intrinsic/n-type/metal stacked structures, aligned with the transparent EWOD electrodes, to detect optical signals generated inside (or modulated by) the liquid droplets handled by the digital microfluidic system.The fabrication process has been designed and performed taking into account the compatibility of all the technological steps of the photosensor and EWOD structures fabrication. The successful integration has been demonstrated checking the correct geometry of EWOD electrodes and measuring the optoelectronic performances of the a-Si:H photosensors at the end of the system fabrication. The correct operation and potentiality of the presented device has been assessed monitoring a photodiode current when a water droplet is moved forward and backward over the EWOD electrodes aligned with the photosensors.

DEMOCHEM: Integrated System for Mycotoxins Detection

In this paper, we present an integrated system to detect Ochratoxin A (OTA) in feed, food and beverage. Its characteristics of portability, simplicity, sensitivity, low cost and user friendly, make the system suitable to be used by unskilled personnel. The system is based on a commercial Thin Layer Chromatography (TLC) plate coupled with an array of amorphous silicon (a-Si:H) photosensors embedded in a PTFE chamber where the chromatographic analysis occurs. The characterization of the system demonstrates its ability to detect OTA contained in wheat, wine and beer with a minimum detectable quantity of 0.2ng and a standard deviation below 5%.

On-chip detection of multiple serum antibodies against epitopes of celiac disease by an array of amorphous silicon sensors

In this paper, we present the preliminary results of an ELISA-on-chip device, intended as a technological demonstrator of a novel analytical system suitable for the diagnosis and follow-up of celiac disease. The idea of the work is to combine an array of amorphous silicon photosensors with a pattern of a poly(2-hydroxyethyl methacrylate) polymer brush film, which acts as anchor for the immobilization of gliadin peptides containing the celiac disease epitopes. Recognition relies on a sandwich immunoassay between antibodies against the peptides and secondary antibodies marked with horseradish peroxidase to obtain a chemiluminescent signal. Detection is based on the measurement of photocurrent induced in the array of amorphous silicon photosensors by the chemiluminescent signal. An ad-hoc procedure has been developed in order to enable the fabrication of the photodiode array and the polymer brush pattern on the two sides of the same glass substrate ensuring the compatibility of the different technological steps. The sensitivity and the selectivity of the chip for multiplex immunoassays were demonstrated using two gliadin peptides (VEA and DEC). In particular, we found that the average amount of the bound HRP revealed by our analytical protocol is 3.5(±0.3) × 10−6 pg μm−2 and 0.85(±0.3) × 10−6 pg μm−2 for specific and non-specific interactions, respectively.

An amorphous silicon photodiode microfluidic chip to detect nanomolar quantities of HIV-1 virion infectivity factor.

A hydrogenated amorphous silicon (a-Si:H) photosensor was explored for the quantitative detection of a HIV-1 virion infectivity factor (Vif) at a detection limit in the single nanomolar range. The a-Si:H photosensor was coupled with a microfluidic channel that was functionalized with a recombinant single chain variable fragment antibody. The biosensor selectively recognizes HIV-1 Vif from human cell extracts.