A nanoporous optofluidic microsystem for highly sensitive and repeatable surface enhanced Raman spectroscopy detection

Abstract

We report the demonstration of an optofluidic surface enhanced Raman spectroscopy (SERS) device that leverages a nanoporous microfluidic matrix to improve the SERS detection performance by more than two orders of magnitude as compared to a typical open microfluidic channel. Although it is a growing trend to integrate optical biosensors into microfluidic channels, this basic combination has been detrimental to the sensing performance when applied to SERS. Recently, however, synergistic combinations between microfluidic functions and photonics (i.e., optofluidics) have been implemented that improve the detection performance of SERS. Conceptually, the simplest optofluidic SERS techniques reported to date utilize a single nanofluidic channel to trap nanoparticle-analyte conjugates as a method of preconcentration before detection. In this work, we leverage this paradigm while improving upon the simplicity by forming a 3D nanofluidic network with packed nanoporous silica microspheres in a microfluidic channel; this creates a concentration matrix that traps silver nanoclusters and adsorbed analytes into the SERS detection volume. With this approach, we are able to achieve a detection limit of 400 attomoles of Rhodamine 6G after only 2 min of sample loading with high chip-to-chip repeatability. Due to the high number of fluidic paths in the nanoporous channel, this approach is less prone to clogging than single nanofluidic inlets, and the loading time is decreased compared to previous reports. In addition, fabrication of this microsystem is quite simple, as nanoscale fabrication is not necessary. Finally, integrated multimode fiber optic cables eliminate the need for optical alignment, and thus the device is relevant for portable and automated applications in the field, including point-of-sample and point-of-care detection. To illustrate a relevant field-based application, we demonstrate the detection of 12 ppb of the organophosphate malathion in water using the nanofluidic SERS microsystem.

Microfluidics and Solid-phase Microextraction Effects in SERS Chips for Rapid and Sensitive Malathion Detection

“…(2). (2) where C w is the malathion concentration in water and D is the diffusion coefficient of malathion molecules.…”

“…But the dilute sample volume and the diffusion-limited transport of analyte molecules within flow channels could be detrimental to the detection limit and baffle the application of optofluidic SERS. Many efforts had been carried out to increase the active concentration of analyte on the surface of nanostructure or nanoparticles substrates using well designed photonic crystal fiber, as well as using active or passive microfluidic techniques such as electrical/magnetic field, nanofluidic channel, nanoporous silica microspheres, and flow-through microhole array [1,2]. By utilizing Solid Phase Micro-Extraction (SPME) effect for pre-concentration, a PDMS microfluidcs chip with quasi-3D (Q3D) gold plasmonic nanostructure arrays, which serve as SERS-active substrates, was experimentally proved to be sensitive and efficient for detecting malathion, a widely used organophosphate insecticide that can contaminate waterways after application in agricultural areas [3,4].…”

Fiber-optic SERS detection enabled by light-induced gold nano-particle aggregation

“…By using the optical tweezer effect, two silver nano-particles could be moved together for SERS detection [9]. Nanoporous silica microsphere matrix was used for enriching analytes in a microfluidic device, and by combining with a two-fiber configuration highly sensitive SERS detection was achieved [10]. Yet, these structure and detection processes were relatively complicated.…”

Surface enhanced Raman spectroscopy (SERS) for in vitro diagnostic testing at the point of care

“…Yazdi and White described another aggregation method by forming a 3D nanofluidic network with packed nanoporous silica microspheres in a microfluidic channel [185]. This matrix trapped silver nanoclusters and adsorbed analytes into the SERS detection area.…”