Spread spectrum Raman spectroscopy for ultrasensitive label-free detection of biomolecules

Abstract

Raman spectroscopy (RS), including surface-enhanced Raman spectroscopy (SERS) serves as a new diagnostic technique for monitoring neurotransmitters at physiologically relevant values: a sensitive technique that provides the structural information of released compounds and spatial information about their release sites. However, highly stable SERS detection still challenges for precise identification of neurotransmitters due to the weak analyte affinity of neurotransmitters and instability of SERS substrates. Conventional SERS studies still struggle for the single molecule-level SERS detection due to a low signal-to-noise ratio and uncertainties in signal intensity. Spread spectrum Raman spectroscopy (ss-RS) is a novel approach for completely resolving the technical barrier of existing SERS or biosensing techniques by applying the spread-spectrum technology well-known in optical telecommunication to the biomolecule detection. This method efficiently removes all the noises including even fluorescence signals. In particular, this method simultaneously encodes SERS signals (lifetime of psec less than the modulation frequency) in the same pattern as the spreading code under spreading coded light excitation. In contrast, fluorescence signals (lifetime of nsec greater than the modulation frequency) are encoded in a distorted pattern and then substantially suppressed by near-zero cross-correlation between the fluorescence signals and the spreading cose. This spread spectrum SERS detection has successfully demonstrated exceptionally high SNR improvement over three orders-of-magnitude by encoding and decoding of spreading codes based on correlation process. The experimental results clearly demonstrate that spread spectrum SERS allows label-free SERS detection of dopamine, serotonin, acetylcholine, GABA, and glutamate neurotransmitters down to the attomolar level. Raman spectroscopy (RS), which can quantitatively analyze distinctive spectral features specific for target molecules, has been developed as a promising method for noninvasive diabetes diagnosis. Tear glucose detection using spread spectrum Raman spectroscopy for noninvasive diabetes diagnosis is experimentally verified. The spread spectrum RS further allows the selective RS detection of glucose in artificial tear down to the nanomolar level. From clinical trial results of human-based tear glucose using ss-RS, we have successfully verified high correlation between tear and blood glucose. The proposed method overcomes the limitation of conventional SERS techniques and provides a new alternative for early diagnosis of neurological disease and real-time and noninvasive diabetes diagnosis. This ultrasensitive performance allows label-free SERS analysis at a single molecular level as well as quantitative molecular diagnostics. Moreover, the spreading coded light excitation can give a new route for developing highly sensitive, high speed, low-cost and handheld biomedical spectroscopic techniques in UV, visible, and infrared even to terahertz ranges. This spread spectrum light excitation driven SERS technique provides a new paradigm for ultrasensitive SERS/RS detection or other ultrasensitive optical biosensing applications. Moreover, this noise-free SERS detection can also serve as a high impact and key tool for label-free small molecule assays including single molecule DNA sequencing, metagenomics, or epigenetic analysis, not only neurotransmitters. The results of this study will make solely technical advances and also have significant impacts on interdisciplinary communities of the following researchers applying cutting-edge technologies in other fields for resolving the technical barrier of the biomolecular detection.