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We present a polymer-coated silicon microring resonator to detect volatile organic compounds (VOC) gases. Absorption of VOC gases into the polymer alters the refractive index around the microring, which enables quantification of absorbed gases as a resonance wavelength shift in the resonator. Two different polymers, hyperbranched polyesteramid and poly(2-vinyl pyridine), are tested as the absorption layers on the highly sensitive (80 nm/RIU and 10,000 Q-factor) silicon microring resonator. The sensing responses to VOC gases are characterized by flowing four different gases: styrene, acetone, toluene, and isoprene, in a wide range of concentration (sub-ppm to 100 ppm range). The hyperbranched polyesteramid coated sensor shows the highest sensitivity to styrene. Similarly, Poly(2-vinyl pyridine) coated device exhibits sensitive responses to styrene and acetone. From experiments with a wide range of gas concentrations, we observe that the sensors have two distinguished characteristics; linear and non-linear response with respect to gas concentration. To account for it, a dual mode sensing mechanism is proposed; Langmuir mode (doping effect) in the low concentration (

MicroRNAs have been identified as promising biomarkers for human diseases. The development of a point-of-care (POC) test for the disease-associated miRNAs would be especially beneficial, since miRNAs are unexpectedly well preserved in various human specimens, including urine. Here, we present the Mach-Zehnder interferometer-miRNA detection system capable of detecting multiple miRNAs in clinical urine samples rapidly and simultaneously in a label-free and real-time manner. Through measurement of the light phase change, the MZI sensor provides an optical platform for fast profiling of small molecules with improved accuracy. We demonstrate that this system could specifically detect target miRNAs (miR-21, and let-7a), and even identify the single nucleotide polymorphism of the let-7 family of miRNAs from synthetic and cell line samples. The clinical applicability of this system is confirmed by simultaneously detecting two types of miRNAs in urine samples of bladder cancer patients in a single reaction, with a detection time of 15 min. The POC system can be expanded to detect a number of miRNAs of different species and should be useful for a variety of clinical applications requiring at or near the site of patient care.

We demonstrate a highly sensitive label-free Mach–Zehnder interferometer (MZI) biosensor based on silicon nitride slot waveguide. Unlike the conventional MZI sensors, the sensing arm of the sensor consists of a slot waveguide while the reference arm consists of a strip waveguide. Thanks to the slot waveguide's property to provide high optical intensity in a subwavelength-size low refractive index region (slot region), which allows high light–analyte interaction, higher sensitivity can be obtained as compared to conventional waveguides using the slot waveguide as sensing region. The bulk refractive index sensitivity of the slot waveguide MZI sensor was found to be 1864π/RIU (refractive index unit) with 7 mm long slot waveguide sensing arm, which shows higher sensitivity compared to the conventional MZI device based on silicon nitride. The biosensing capability of the developed slot waveguide MZI was investigated using biotin–streptavidin binding as a model system. The sensitivity of the system was demonstrated down to 18.9 fM or 1 pg/ml of streptavidin solution and to the best of our knowledge, it is the best reported experimental value for the limit of detection of a MZI sensor. Furthermore, we investigated the specific detection and quantification of the methylation of DAPK (Death-associated protein kinase) gene, which is a widely used biomarker for human cancers. We have shown that methylation sequences of DAPK gene of various methylation densities (100%, 50%, and 0% of methylation sites) can be quantified and discriminated even at a concentration as low as 1 fmol/μl or 1 nM. ASTAR (Agency for Sci., Tech. and Research, S’pore)

We present a novel biosensor based on an electrical tracing-assisted silicon dual-microring resonator sensor system. The dual-microring system comprises one microring resonator as a sensing element and the other microring resonator integrated with an electrical controller as a tracing element. The resonance wavelength shift of the sensing microring induced by the refractive index change due to antigen-ligand bindings is traced and determined by direct voltage applied to the electrical tunable tracing microring. The sensor system enables the use of a low-cost broadband light source instead of a bulky and expensive tunable laser, which allows the development of cost-effective point-of-care diagnostic devices by significantly reducing the device cost and increasing its portability. The sensing capability of the developed dual-microring sensor was investigated using biotin-streptavidin binding as a model system. We have demonstrated the quantitative detection of streptavidin over a broad range of concentrations down to 190 pM by monitoring the electrical power applied to the tracing ring. We have also validated the sensing principle of the dual-microring system by a direct comparison between the calculated and measured values for the resonance wavelength shift of the sensing microring. Furthermore, we have shown the quantitative and specific detection of a well-known breast cancer biomarker, human epidermal growth factor receptor 2 (HER2), in a bovine serum albumin solution using the antibody-modified dual-microring sensor system.

Detection and quantification of DNA methylation is important to provide an opportunity to resolve clinical issues such as cancer early detection, progression, and drug treatments. In spite of myriad current technologies for DNA methylation detection that have been developed in last decade, they are still complicated and inadequate to be used as in vitro clinical diagnostic device. This work presents the first methylation specific sensor based on the silicon microring resonators which achieves fast, simple and specific label-free detection of DNA methylation after the bisulfite conversion. The methylated promoter regions of three genes ( DAPK , E - cadherin and RARβ ), which have been widely studied as biomarkers for human cancers including bladder cancer, are used as the target DNA sequences. We show that the methylated targets are strongly captured by methylated probe compared to unmethylated probe or vice versa. The discrimination between methylated and unmethylated DNA sequences is achieved within 5 min after hybridization between target and probe. Additionally, we have quantified DNA methylation density using various proportions (100, 75, 50, 25, and 0% of methylation sites) of methylation sequences of DAPK gene. Finally, we confirmed that the sensor can clearly detect the methylation of RARβ gene by using amplified target from genomic DNA of cancer cells. Therefore, our technique can be used to detect and quantify the methylation density in cancer biomarkers.

A monolithic integration of poly(dimethylsiloxane) (PDMS) waveguides and microfluidics is demonstrated by implementing a two-step lithographic moulding. Silicone oil is added in PDMS precursor to tune the refractive index of waveguides with air as the side and top cladding layer. Curved and straight waveguides are characterized for their insertion loss mainly due to propagation, radiation, and transition. Transition and radiation losses are identified dominating along the curves whereas propagation loss increases at a steady rate of 0.249 ± 0.015 dB/cm. With the integrated system, real-time absorbance measurement of fluorescein streams yields a nearly linear response up to 250 μM and the limit of detection