The field of microfluidics has yet to develop practical devices that provide real clinical value. One of the main reasons for this is the difficulty in realizing low-cost, sensitive, reproducible, and portable analyte detection microfluidic systems. Previous research has addressed two main approaches for the detection technologies in lab-on-a-chip devices: (a) study of the compatibility of conventional instrumentation with microfluidic structures, and (b) integration of innovative sensors contained within the microfluidic system. Despite the recent advances in electrochemical and mechanical based sensors, their drawbacks pose important challenges to their application in disposable microfluidic devices. Instead, optical detection remains an attractive solution for lab-on-a-chip devices, because of the ubiquity of the optical methods in the laboratory. Besides, robust and cost-effective devices for use in the field can be realized by integrating proper optical detection technologies on chips. This review examines the recent developments in detection technologies applied to microfluidic biosensors, especially addressing several optical methods, including fluorescence, chemiluminescence, absorbance and surface plasmon resonance.

No abstract

A general expression is given for the change in free energy when a charge tunnels through a junction in a one-dimensional array of N metallic islands with arbitrary capacitances and arbitrary background charges. This is used to obtain expressions for the ͑average͒ threshold voltage of the Coulomb blockade for a few characteristic geometries. We find that including random background charges has a large effect on the N dependence of the threshold voltage: In an array with identical junction capacitances C and gate capacitances C g , the threshold voltage, averaged over the background charge, is proportional to N a , where a crosses over from 1 2 to 1 when N becomes larger than 2.5ͱC/C g .

A picogram-sensitive optical microfluidic biosensor using an integrated polycarbazole photodiode is developed. The photodetector is mainly composed of the blend heterojunction of poly [N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester (PC70BM) and the poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) as the hole transport layer. Analyte detection is accomplished via a chemiluminescent immunoassay performed in a poly(dimethylsiloxane)-gold-glass hybrid microchip, on which antibodies were immobilized and chemiluminescent horseradish peroxidase-luminol-peroxide reactions were generated. Enhanced sensor response to the chemiluminescent light is achieved by optimizing the thickness of PCDTBT:  PC70BM and PEDOT:PSS. Using the optimized polycarbazole photodiode for detecting the human thyroid-stimulating hormone as the model target, the integrated biosensor demonstrates an excellent linearity in the range of 0.03 to 10  ng/ml with an analytical sensitivity of 68  pg/ml. The sensor response shows high specificity and reproducibility. Hormone detection in clinical samples is further demonstrated and compared with a commercial enzyme-linked immunosorbent assay. The integrated device reported here has potential to detect other hormonal compounds or protein targets. Moreover, the presented concept enables the development of miniaturized, low-cost but highly sensitive optical microfluidic biosensors based on integrated polymer photodetectors with high potential for point-of-care diagnostics.

No abstract