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.

An analytical expression for shot noise in a correlated sequential tunneling current has been derived by solving the master equation exactly, with the Pauli correlation as a special case. Our theory is applied to the Coulomb-blockade single-electron tunneling system with two tunnel junctions. Given capacitances and resistances of the system, the temperature dependence and the bias-voltage dependence of the noise spectrum in the entire range of frequency have been analyzed in detail. We have demonstrated that the noise spectrum is more sensitive to the bias voltage than is the current itself to the voltage. Therefore, noise spectroscopy is a much more accurate tool to investigate the correlation in tunneling transport through ever smaller multiple-junction structures.

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 C ₇₁ -butyric acid methyl ester (PC ₇₀ BM ) 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:  PC ₇₀ BM 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.

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 (average) 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 ${\mathrm{C}}_{\mathrm{g}}$, the threshold voltage, averaged over the background charge, is proportional to ${\mathrm{N}}^{\mathrm{a}}$, where a crosses over from to 1 when N becomes larger than 2.5$\sqrt{C/C_g}$.

This paper presents a new approach for the design of film bulk acoustic resonators (FBARs) with high-quality factor Q _a at antiresonant frequency f _a . Lamb waves are among the main contributors to the loss of energy at f _a and the deterioration of Q _a . The targeted approach is an optimization process of a dual-step frame structure to alleviate the problem with Lamb waves at f _a . Such a frame is a combination of two neighboring steps that have different acoustic impedances reflecting the two Lamb modes that carry the largest amount of energy. The dimensions of these steps are computed using the diffraction grating method. To do so, the dispersion curves of Lamb waves and their associated power flows are calculated using a global matrix method and the acoustic Poynting's Theorem. This results in an enhancement in Q _a of nearly 82% for an FBAR with a dual-step frame in comparison with an FBAR with no frame, as well as a smoother electrical response, at frequencies above fa but below the resonant frequency f _r spurious resonances are moderately increased.

Our previous work on finite-frequency shot noise S(ω) in a correlated tunneling current has been extended to investigate the shot-noise behavior in a single-electron transistor (SET), with emphasis on asymmetric SET. We found that both the increase of the asymmetry and the increase of the gate voltage will broaden S(ω), and S(0) is further suppressed by the gate capacitance. At low bias voltage, a condition is obtained under which the noise spectrum S(ω) is white (frequency-independent). We have also demonstrated at the quantitative level how the signal-to-noise ratio in a SET can be controlled by adjusting the gate voltage and the degree of asymmetry.