R. Scipinotti

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.

Microfluidic Chip With Integrated a-Si:H Photodiodes for Chemiluminescence-Based Bioassays

On-chip optical detection of chemiluminescent reactions is presented. The device is based on the integration of thin film hydrogenated amorphous silicon photosensors on a functionalized glass substrate ensuring both a good optical coupling and an optimal separation between biological or chemical reagents and the sensing elements. The sensor has been characterized and optimized using the chemiluminescent system composed by the enzyme horseradish peroxidase (HRP) and luminol/peroxide/enhancer cocktail. The detectability of HRP is at the attomole level with a sensitivity of 1.46 fA/fg. Experiments, involving the detection of immobilized bio-specific probes on the functionalized surface have been performed both in bulk and microfluidics regime, proving the ability of the system to effectively detect chemiluminescent reactions and their kinetics. In particular, results achieved using conventional polydimethylsiloxane microfluidics for samples and reagents handling confirmed the good detection capabilities of the proposed system.

Monitoring of Temperature Distribution in a Thin Film Heater by an Array of a-Si:H Temperature Sensors

In this paper, we propose the use of an array of amorphous silicon (a-Si:H) p-i-n diodes to monitor the spatial temperature distribution over a thin film heater used for thermal treatments in lab-on-chip systems. The effects of heater geometry and operating conditions on the spatial temperature distribution have been preliminarily investigated by using COMSOL Multiphysics, coupling the electrostatic problem with the thermal problem via the Joule effect. Depending on the analyzed system, nonuniform temperature profiles can be induced over the heater surface revealing the need for a temperature point-monitoring. An example of whole device, constituted by a serpentine shaped TiW/Al/TiW thin film heater and five a-Si:H diodes deposited between the resistor meanders, has been fabricated on a microscope glass slide and characterized. Voltage-temperature characteristics of the a-Si:H sensors, measured at constant forward current, show a sensitivity around . The spatial temperature distribution along the heater has been derived measuring the voltage across each a-Si:H diode. A good agreement between modeled and measured data is obtained, demonstrating the suitability of the a-Si:H array as temperature distribution sensors in lab-on-chip application.

Amorphous Silicon Sensors for Single and Multicolor Detection of Biomolecules

In this paper, we report on a system for single and multicolor detection of biomolecules based on amorphous silicon photosensors. The system promises to be compact, portable, and low cost. It allows the quantitative detection without using optics for focusing both the excitation and the emitted radiation. The revealed biomolecules can be chemi- or naturally luminescent or can be labeled with fluorochromes. Here, we focus on the detection of DNA molecules labeled with a single or with two fluorochromes by using a p-i-n and a p-i-n-i-p amorphous silicon stacked structure, respectively. The device design has been optimized in order to maximize the signal-to-noise ratio and to match the sensor spectral response with the emission spectra of the fluorochromes. This optimization process has been carried out by means of a numerical device simulator, which takes into account the optical and electrical properties of the amorphous silicon. Detection limit in the order of a few nmol/l have been achieved for both the single and the two-color photosensors. Comparison with commercial measurement equipment shows the suitability of our system for practical applications.

Amorphous Silicon Photosensors for Detection of Ochratoxin a in Wine

The presence of ochratoxin A (OTA) in different food commodities deserves great attention because of its toxic and carcinogenic effects on humans and animals. In this paper, we report on the detection of OTA both in standard solutions and in contaminated samples of red wine by using amorphous silicon photosensors. The method relies on the excitation by ultraviolet radiation of the toxin molecules and on the absorption of the toxin reemitted light by the photosensor. The device is a p-i-n stacked structure, whose electro-optical characteristics have been optimized in order to maximize the photosensor responsivity and the limit of detection. For standard solutions, we found minimum detected OTA amount to be equal to 0.1 ng, while for contaminated red wine samples the technique coupled with very simplified and rapid extraction procedures has allowed the detection of OTA at the 1-ppb level.

Lab-on-glass system for DNA analysis using thin and thick film technologies

In this paper, we present a compact lab-on-chip system suited for label-free DNA analysis. The system can be fabricated on a conventional microscope glass slide using thin-film and thick-film technologies. It integrates a heating chamber, an electrowetting-based droplet handling system and a hydrogenated amorphous silicon (a-Si:H) photosensor array for DNA detection. At this stage of research we have designed and tested the individual functional units. The heating chamber incorporates a thin metal film heater optimized for uniform temperature distribution on a 1cm 2 area. A forward-biased a-Si:H p-i-n junction is used for temperature monitoring, achieving a linear temperature dependence with -3.3 mV/K sensitivity. The droplet-handling unit, relying on the electrowetting method, is designed to move the sample from the heating chamber to the sensor array. The unit includes a set of metal pads beneath a layer of PDMS that provides both the electric insulation of the electrodes and the hydrophobic surface needed by the electrowetting technique. The UV sensor array allows measuring the DNA absorbance variation at 254nm related to the hybridization between probe-molecules contained in the sample and reference target molecules immobilized on the sensor surface. A preliminary test to detect the hybridization between a 25-mer single-stranded oligonucleotides and denaturated pBR 322 4162-mer single-stranded oligonucleotides has been carried out successfully.

On-Chip Diagnosis of Celiac Disease by an Amorphous Silicon Chemiluminescence Detector

A lab-on-chip for the diagnosis of celiac disease relying on the monitoring of patient-specific immune response to gliadin fractions has been developed. The detection is based on a chemiluminescent immunoenzymatic reaction that ensures high specificity and sensitivity. The chemiluminescent signal is monitored by hydrogenated amorphous silicon photosensors, fabricated on the same glass substrate hosting the biochemical recognition. The main challenge of the work has been the identification of the materials and the setup of the entire process that permitted the reliable fabrication of the device. Experiments performed with serum samples of rabbit immunized towards an epitope show a good specificity of the proposed technique, proving the feasibility of an integrated device for the patient-specific profiling.

Electrowetting-on-dielectric system based on polydimethylsiloxane

In this paper we present a detailed characterization of an electro-wetting on dielectric (EWOD) system able to move drop of liquid and to detect its position over an array of electrodes covered with a 1μm thick polydimethylsiloxane (PDMS) layer. In the presented system, the PDMS layer acts as both insulation and hydrophobic material. An electronic board controls all the signals needed for the actuation and sensing functionalities of the EWOD system. Sessile drop experiments show the saturation of the contact angle at negative bias voltage applied to the droplet. This behavior is ascribed to trapped carrier in the PDMS layer and explains the movement of the droplet toward the grounded electrode found in EWOD experiment. The procedure chosen for the drop movement achieves speed around 5cm/s with applied voltages around 200V. Detection of drop position is successfully achieved implementing the time-constant method, which evaluates the variation of electrode capacitance induced by the droplet presence on the PDMS surface corresponding to the metal electrode.

Modeling of the photo-response of a smart thin layer chromatography system

Smart thin layer chromatographic plates integrate thin film photo-sensors for the in-situ real-time monitoring of the chromatographic run, enabling additional functionalities such as precise control of the plate development and additional analytical information on the sample. In order to extract useful information in real-time during the development of a chromatographic plate with integrated photo-sensors an analytical model is proposed based on geometrical optics considerations. The model is then used on experimental data to exemplify its use for gathering additional analytical information on the sample.

Chemiluminescence-Based Micro-Total-Analysis System with Amorphous Silicon Photodiodes

In this paper we present a chemiluminescence-based micro-total-analysis system integrating amorphous silicon for on-chip detection as a technically feasible solution to develop “true” lab-on-chip systems, intended as stand-alone devices implementing all the analytical steps from sample preparation to on-chip detection. The achieved performances are comparable to that of the state-of-the-art lab equipment demonstrating that such systems would enable the development of a variety of point-of-care testing systems, opening new analytical application scenarios.