Recent advances in label-free biosensing techniques have shown the potential to simplify clinical analyses. With this motivation in mind, this paper demonstrates for the first time the use of siliconon-insulator microring optical resonator arrays for the robust and label-free detection of a clinically important protein biomarker in undiluted serum, using carcinoembryonic antigen (CEA) as the test case. We utilize an initial slope-based quantitation method to sensitively detect CEA at clinically relevant levels and determine the CEA concentrations of unknown samples in both buffer and undiluted fetal bovine serum. Comparison with a commercial enzyme-linked immunosorbent assay (ELISA) kit reveals that the label-free microring sensor platform has a comparable limit of detection (2 ng/mL) and superior accuracy in the measurement of CEA concentration across a three order of magnitude dynamic range. Notably, we report the lowest limit of detection to date for a microring resonator sensor applied to a clinically relevant cancer biomarker. Although this report describes the robust biosensing capabilities of silicon photonic microring resonator arrays for a single parameter assay, future work will focus on utilizing the platform for highly multiplexed, label-free bioanalysis.

Due to the inherent complexity of, and interactions between, biochemical pathways commonly altered in disease states, it has become accepted that multiplexed analyses can provide a more informative biomolecular understanding of disease onset and progression. Importantly, compared to conventional single-parameter assays, the detailed biomolecular insight gleaned from multiparameter measurements has the potential to greatly improve disease diagnostics, prognostics, and theragnostics. We have previously reported the utility of silicon photonic microring resonators for the sensitive quantitation of a single disease biomarker and herein demonstrate the first example of optical microcavity resonator arrays performing quantitative, and label-free, multiplexed analyses of clinically relevant protein biomarkers. In this report, the concentrations of prostate specific antigen (PSA), α-fetoprotein (AFP), carcinoembryonic antigen (CEA), tumor necrosis factor alpha (TNF-α), and interleukin-8 (IL-8) are simultaneously determined in three unknown protein cocktail solutions. This paper demonstrates that multiple immunoassays can be performed concurrently on a microresonator platform without any accompanying loss of sensitivity or measurement precision, and, therefore, this report lays the groundwork for future applications involving multiplexed analysis of clinically-relevant samples.

By leveraging advances in semiconductor microfabrication technologies, chip-integrated optical biosensors are poised to make an impact as scalable and multiplexable bioanalytical measurement tools for lab-on-a-chip applications. In particular, waveguide-based optical sensing technology appears to be exceptionally amenable to chip integration and miniaturization, and, as a result, the recent literature is replete with examples of chip-integrated waveguide sensing platforms developed to address a wide range of contemporary analytical challenges. As an overview of the most recent advances within this dynamic field, this review highlights work from the last 2–3 years in the areas of grating-coupled, interferometric, photonic crystal, and microresonator waveguide sensors. With a focus towards device integration, particular emphasis is placed on demonstrations of biosensing using these technologies within microfluidically controlled environments. In addition, examples of multiplexed detection and sensing within complex matrices—important features for real-world applicability—are given special attention.

In the post-genomic era, information is king and information-rich technologies are critically important drivers in both fundamental biology and medicine. It is now known that single-parameter measurements provide only limited detail and that quantitation of multiple biomolecular signatures can more fully illuminate complex biological function. Label-free technologies have recently attracted significant interest for sensitive and quantitative multiparameter analysis of biological systems. There are several different classes of label-free sensors that are currently being developed both in academia and in industry. In this critical review, we highlight, compare, and contrast some of the more promising approaches. We will describe the fundamental principles of these different methodologies and discuss advantages and disadvantages that might potentially help one in selecting the appropriate technology for a given bioanalytical application.