18.03.2021 •

In Situ Integration of SERS Sensors for On‐Chip Catalytic Reactions

The rapid progress of microfluidics has contributed to modern analytical methodology. However, in situ integration of chemical/biological sensors for on‐chip reaction monitoring remains a big challenge. Here, a graphene‐based catalytic chip with surface‐enhanced Raman scattering (SERS) sensors was fabricated by laser scribing of a graphene oxide (GO)–silver nanoparticle (AgNP) composite (AgNPs@GO). The surface plasmonic effect of AgNPs promoted the photoreduction of GO (RGO), inducing a highly porous nanostructure. By combining the AgNPs@RGO composite patterns with polydimethylsiloxane microfluidic channels, a bifunctional microfluidic chip was fabricated capable of on‐chip catalysis and SERS detection. As a proof‐of‐concept demonstration, on‐chip AgNP‐catalyzed 4‐nitrophenol reduction was performed while the reaction process was monitored by the in situ SERS sensor. The integration of SERS sensors with functional microfluidic devices holds great promise for developing advanced Lab‐on‐a‐Chip systems.

Introduction

Lab‐on‐a‐Chip (LoC) systems are highly integrated devices allowing multiple experimental procedures, including sample injection, reaction handling, product separation, and component analysis, on a single chip [1]. The ultra‐low consumption of fluids makes LoC favorable in medical diagnostics, environmental monitoring, and chemical analysis [2–4]. To further extend the functionalities of LoC devices, various functional components have been successfully integrated with microfluidic chips, which significantly facilitated automatic analysis [5]. Nevertheless, most of these proof‐of‐concept devices cannot be applied in real production as the fabrication of these multifunctional LoCs generally involves complex manufacturing procedures [6]. The channel networks need to be specially designed, and the on‐chip fabrication techniques are usually highly precise 3D nanotechnologies that are not yet fully developed for market‐oriented commercialization. Therefore, realizing multi‐functionalization on a single chip remains a big challenge. 

Laser scribing technology that converts near‐infrared light (NIR, ≈780 nm) inside a digital versatile disc (DVD) drive into localized heat is a cost‐effective and simple tool for flexible patterning of graphene and many composite materials [7]. Laser‐scribed graphene oxide (GO) has been widely used to produce electronic devices because of its good mechanical properties, high electrical conductivity, and high surface area [8]. Thus, laser scribing of GO is promising for fabricating graphene‐based chemical sensors, actuators, and various electronics, which may contribute to the development of microfluidics. However, laser scribing of GO has not been fully applied to LoC functionalization. 

In this study, a surface‐enhanced Raman scattering (SERS)‐active catalytic chip was fabricated by laser scribing of GO/silver nanoparticle composites (AgNPs@GO). The surface plasmonic effect of AgNPs promoted the photoreduction of GO, leading to a highly porous nanostructure. The well‐exfoliated graphene served as an excellent catalytic scaffold for AgNPs. The dense distribution of AgNPs on the laser‐scribed graphene led to a plasmonic structure, making the microfluidic chip SERS-active. Combining a catalyst bed with the SERS substrate enabled on‐chip catalysis and in situ SERS monitoring of 4-nitrophenol (4-NP) reduction. Thus, integrating SERS sensors with functional microfluidic devices holds great promise for developing advanced LoC systems in the future.

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