Preparation and application of microfluidic SERS substrate: challenges and future perspectives

Journal Pre-proof Preparation and application of microfluidic SERS substrate: challenges and future perspectives Jiuchuan Guo, Fanyu Zeng, Jinhong Guo, Xing Ma

PII:

S1005-0302(19)30296-8

DOI:

https://doi.org/10.1016/j.jmst.2019.06.018

Reference:

JMST 1704

To appear in: Received Date:

24 February 2019

Revised Date:

17 June 2019

Accepted Date:

18 June 2019

Please cite this article as: Guo J, Zeng F, Guo J, Ma X, Preparation and application of microfluidic SERS substrate: challenges and future perspectives, Journal of Materials Science and amp; Technology (2019), doi: https://doi.org/10.1016/j.jmst.2019.06.018

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Prospective/Viewpoint Preparation and application of microfluidic SERS substrate: challenges and future perspectives Jiuchuan Guoa, Fanyu Zengb,c, Jinhong Guoa,*, Xing Mab,c,*

a

School of Information and Communication Engineering, University of Electronic Science and

b

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Technology of China, Chengdu 610054, China

State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology

(Shenzhen), Shenzhen 518055, China. c

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Flexible Printed Electronic Technology Center, School of Materials Science and Engineering,

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Harbin Institute of Technology (Shenzhen), Shenzhen 518055, China

*

Corresponding authors.

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E-mail addresses: [email protected] (J. Guo); [email protected] (X. Ma). [Received 24 February 2019; Received in revised form 17 June 2019; Accepted 18

Abstract

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June 2019]

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Surface-enhanced Raman spectroscopy (SERS), as a highly sensitive molecular analysis technique, can realize fast and non-destructive detection of the information of

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molecular bonds to identify the component of analytes by “fingerprint” identification. The preparation of SERS substrates plays an extremely important role in the development of SERS technology and the application of SERS detection. By integrating SERS enhancement substrates into microfluidic chips, researchers have developed the microfluidic SERS chips which expand the function of microfluidic chips and provide an efficient platform for on-site biochemical analysis equipped with the powerful sensing capability of SERS technique. In this paper, we will first briefly 1

give a review of the current microfluidic SERS-active substrates preparation technology and present the perspective on the application prospects of microfluidic SERS-active substrates. And then the challenges in the preparation of microfluidic SERS-active substrates will be pointed out, as well as realistic issues of using this technology for biochemical application.

Keywords: SERS substrate; Microfluidics; Biochemical sensing; On-site analysis

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1. Introduction

In recent years, spectroscopy technologies have attracted many interests due to the advantages of highly sensitive, fast, efficient and non-destructive imaging [1]. Among

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them, Raman spectrum is related to the molecular vibrational energy level and can

provide unique "fingerprint" information for each molecule, which makes Raman

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spectroscopy very convenient and efficient in studying the composition of substance. However, the intensity of the Raman scattering signal of the target analytes is

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generally low, resulting in low sensitivity of the Raman scattering spectroscopy. In 1974, Fleischmann et al. discovered that rough metal surfaces can greatly enhance

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Raman scattering with a magnification of 103 to 104 [2]. This phenomenon is known as the surface enhanced Raman scattering (SERS) effect. The discovery of SERS had a profound impact on the development of Raman spectroscopy. In the SERS-active

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substrates, highly localized regions of intense local field enhancement caused by localized surface plasmon resonance (LSPR) are referred to as "hot spots"[3], which

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result in extraordinary enhancements to the SERS signal [4]. Ding et al. [5] divided the development of "hot spots" into three generations. The first-generation "hot spots" are made from metal nanoparticles with single

geometrical nanostructures. Their sharp corners or intraparticle gaps can form effective "hot spots". Spherical gold nanoparticles, rod-shaped gold nanoparticles, star gold nanoparticles, and etc. are all typical representatives. The second generation of 2

SERS "hot spots" are generated from nanoparticle assembly that creates "hot spots" by extremely strong electromagnetic field from coupled nanostructures, such as the nanoparticle dimers, nanoparticle arrays or interunit nanogaps in nanopatterned surfaces. The obtained SERS signal intensity can be 2‒4 orders of magnitude greater than the first generation [6]. The third generation can be considered as "hot spots" right on the probe surface of the materials by developing plasmonic nanostructures. Nano-pillar arrays [7], SHINERS arrays [8], and nanosphere lithography (NSL) arrays [9] all belong to the third-generation SERS substrates, which provide more uniform

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and intense SERS enhancement [10]. After more than 40 years of development, the SERS mechanism research has made great progress, providing a theoretical basis for

the preparation and application of the highly ordered and highly sensitive SERS-active substrates.

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Meanwhile, microfluidic chips have a growing trend in the development of biochemical analysis and detection. The microfluidic system is called "lab on a chip"

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(LoC) [11], and has the advantages of low sample consumption, short reaction time, high detection efficiency and portability. Microfluidic SERS chip (SERS-LoC) is

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formed by injecting or integrating a SERS-active matrix into a microfluidic platform to combine SERS with microfluidic chips, and is currently used for portable sensing

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techniques, such as detecting drugs, oligonucleotides, biomolecules, and proteins [12]. On the other hand, LoC systems could serve as an integrated platform to make SERS detection more reproducible, efficient, safe, and environmentally friendly [13]. For

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example, solid-state SERS substrates deposited on the microfluidic channels enables microfluidic chips to be more reusable by reducing “memory effect” [14].

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Furthermore, highly integrated microfluidic systems make it possible for the emergence of the portable Raman device for on-site detection of the environment and food safety without any contamination and waste [15]. Due to the small size of the LoC systems, the researchers have made a lot of exploration on the preparation and the integration between the SERS enhancement substrates and the microfluidic channels to achieve high sensitivity and high reproducibility of the SERS chips [16]. 3

This paper will first briefly give a review of the development of microfluidic SERS-active substrates preparation technology, and then discuss and comment on the up-to-date application prospects of microfluidic SERS substrates. At last the perspective on the future challenges of microfluidic SERS-active substrates will be given, in order to give suggestions and the future direction of the development of the microfluidic SERS chips.

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2. Microfluidic SERS substrate preparation technology

By introducing SERS substrates into the microfluidic channel, quantities of scientific research has shown that the microfluidic chip has good compatibility with

SERS [17‒19], and the combination of SERS and microfluidic system has made great

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contribution to broadening their application fields. As SERS substrates are the key to

the testing of microfluidic SRES chips, the research on microfluidic SERS-active

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substrates has become an important research topic and developed rapidly in recent years [20,21], in order to realize microfluidic SERS chips with fast detection, high

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sensitivity, good repeatability and high integration.

At present, one method of integrating SERS into the microfluidic chip to realize

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SERS test is to use colloid-based nanoparticle as SERS-active substrate. The colloidal nanoparticle is injected into microfluidic channel and mixed with analyte molecular solution to perform SERS test on the analyte molecules irradiated by the excitation

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laser. However, the interaction of analyte molecules with the colloidal nanoparticles inside a conventional microfluidic channel is governed by the slow diffusion forces,

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and this leads to high signal variations and thus to low reproducibility [22]. In order to overcome this problem, Yazdi et al. [23] designed a surface enhanced resonance Raman spectroscopy (SERRS)-based competitive displacement assay in an integrated microfluidic chip to realize sensitive and multiplexed detection of DNA sequences, as is shown in Fig. 1. The platform evolved from a single microchannel chip, where the analyte and colloidal nanoparticles were mixed off-chip and then injected via the 4

same port into the system. Displacement scheme is used in the experiment, where a Raman-labeled reporter sequence is displaced by target DNA sequence, which enables detection of unlabeled target DNA sequences with a simple single-step procedure. For on-chip multiplexed DNA sequence detection using SERRS, two batches of silica beads are pre-functionalized with probe-reporter pairs and the displacement reaction occurs in it. When the displaced reporter flows downstream in the microfluidic channel, they are mixed with metal nanoclusters and trapped in the microfluidic SERRS detection region. The experimental results reported the detection down to 100

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pM of the target DNA sequence, and the experiments are shown to be specific, repeatable, and quantitative.

However, there are some limitations of colloid-based nanoparticles substrate. Firstly, the thorough mixing of the colloidal nanoparticle with the sample molecules

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takes a long time, resulting in rather low SERS detection efficiency and potential sample contamination. Besides, the analyte–nanoparticle conjugates will get enriched

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over time on microfluidic channel, which can lead to the so-called memory effect [14] and thus to make microfluidic devices less repeatable.

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These drawbacks can be overcome by immobilizing metallic nanoparticle or nanostructures with elaborate morphologies on the surface of the microchannel as

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solid-state SERS-active substrates. The sample molecules in the microchannel may only generate SERS signals when adsorbed to the SERS-active nanostructures during the detection process, avoiding the difficulty of separation between analytes and

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nanomaterials, thus resulting in better reusability of the SERS chip. Meier et al. [24] designed an SERS chip based on ink-jet printed silver nanoparticles on ITO electrodes

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as the SERS enhancement substrate. The chip utilizes the conductivity of ITO. An electrical potential is applied to promote analyte desorption of substrate adsorption. This allows the SERS substrate to be quickly and efficiently regenerated, avoiding substrate memory effects and making the chip reusable. Furthermore, the emergence of droplet microfluidics also helps to resolve the problem of memory effects. Jeon et al. [25] proposed an SERS-based gradient droplet microfluidic system to 5

simultaneously monitor chemical and biological reactions for various concentrations of a reagent (Fig. 2). The research allows minimization of resident time distributions of reagents by diluting the reagents in a stepwise manner and then injecting the oil mixture into the channels to form various concentrations of reagents, which are simultaneously trapped into the tiny volume of droplets in different channels. Thus, the sample stacking problem could be solved in this system and the memory effect is greatly reduced. In recent years, in situ synthesis of SERS substrates on microfluidic chips

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technology has been developed as a better integration method between SERS detection and microfluidic technique. Owing to precise control of the liquid in the microfluidic system, in situ synthesis technology enables SERS substrate fabrication

to be more controllable and flexible. And this opens a new door for high-sensitivity

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and on-site detection of microfluidic SERS chips. Xu et al. [26] present an on-chip fabrication of silver microflower arrays (SMAs) constructed by upright nanoplates

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and attached nanoparticles via femtosecond laser induced photo-reduction of a silver precursor to realize in situ SERS monitoring of catalytic reduction (Fig. 3). SMAs act

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as robust catalytic active sites to promote catalytic reduction on the chip. For the first time, the whole catalytic reduction process can be in situ monitored by SERS

enhancement.

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detection on the microfluidic chip, since the silver substrates exhibit high SERS

Yan et al. [27] researched the method of two-step photo-reduced SERS materials by

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focus on Ag+ ions with the laser spot to produce silver nanoparticle aggregates in the microfluidic channel (Fig. 4). These microfluidic SERS materials were found to have

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detection limit concentrations of detectors such as crystal violet (CV) rhodamine 6G (R6G), methylene blue as low as 10-13 M. In order to make the SERS detection process more controllable, Han et al. [28] demonstrated an SERS biochip based on silver nanoparticles (AgNPs) and graphene oxide (GO) for DNA detection by using facile laser scribing method. SERS-active patterns can be directly written on the microfluidic channel to form a biochip for SERS detection due to the programmable 6

laser scribing of the AgNPs@GO composite film. The non-covalent interaction between DNA and graphene materials results in controllable capture and release of DNA sequences, enabling efficient on-chip DNA detection and biochip regeneration. The results show that the biochip can realize the on-chip SERS detection of a 30-base ssDNA samples at 10-6 M within 1 min with reasonable high SERS signal reproducibility. Despite the advances on the in-situ fabrication of SERS substrates in recent years, the SERS chips still suffer from hard real-time sensing in biometrics. Leem et al. [29]

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demonstrate a chemical reduction method to synthesize nanostructured films directly on the PDMS microfluidic channel via ethylene glycol. Under the optimal conditions, real-time SERS detection of bimolecular adenosine with different concentrations can

be realized. However, the nanostructure integrated by this method generally has poor

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uniformity and causes serious signal interference, which limits its application in realistic scenario. Considering the above problems, Bai et al. [30] proposed a novel

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all-femtosecond-laser-processing technique for the in-situ fabrication of 2D periodic metal nanostructures inside 3D glass microfluidic channels and combined it with

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study,

the

3D

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SERS, where toxic substances can be sensed in real time with ultra-high sensitivity. In glass

microfluidic

channel

is

fabricated

by

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femtosecond-laser-assisted wet etching. And femtosecond laser direct writing ablation and electroless metal plating enables in-situ space-selective formation of Cu-Ag layered films inside the microfluidic structure. Subsequent irradiation with a linearly

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polarized beam causes the Cu-Ag film to form a periodic surface nanostructure. The fabrication process of the microfluidic chip is described in detail in Fig. 5. The

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experimental results show that the obtained 3D microfluidic chip is capable of the real-time SERS detection of Rhodamine 6G, exhibiting an enhancement factor of 7.3 × 108 in conjunction with a relative standard deviation of 8.88%, and Cd2+ ions at concentrations as low as 10 ppb in the presence of crystal violet.

3. Application prospect of microfluidic SERS substrates 7

New technologies lead to new applications. Microfluidic systems based on SERS-active substrates are having a promising application prospect including biomedical sensing, environmental monitoring, and food safety detecting. In order to illustrate the application of the microfluidic SERS more clearly, Table 1 briefly shows the application range, the cutting edge applications and current status of the microfluidic chips based on SERS substrates. The non-destructive properties of SERS-LoC make it ideal candidate for

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biomedical sensing and/or diagnosis. Some biomolecules (such as antigen/antibody, biotin/avidin and DNA) can specifically bind to the target molecule and form a stable SERS-active complex through complementation, stacking principles or hydrogen

bonding. Coupling these biomolecules with microfluidic SERS substrates, SERS-LoC

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has made outstanding advances in bioassays with high sensitivity. Fu et al. [31] constructed a rigid assembly structure of silver nanoparticles on the silver film

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substrate with the aptamer of a DNA enzyme segment which is sensitive to lead ions. Based on the catalytic cleavage reaction of lead ions on the segment, the supported

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aptamer turns into a flexible segment to form the SERS "hot spots" structure, which achieves high sensitivity detection of lead ions. Willner et al. [32] presented the first

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application of SERS droplet microfluidics for single-cell analysis. This experiment encapsulates single prostate cancer cells and wheat germ agglutinin (WGA) functionalized SERS nanoprobes in oil-in-water droplets using a microfluidic device,

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as shown in Fig. 6, and then locked into a droplet array. The "slower" detailed interrogation of the hotspots identified on the fixed droplets enables rapid

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identification of SERS regions of interest in live cancer cells. The results of the experiment examined the expression of glycans on the surface of prostate cancer cells, and demonstrated the potential of SERS optical flow control system for high-throughput cell screening, and illustrated the difference in size and number of glycan islands between cells. Problems still exist in the field of microfluidics SERS technique for biomedical 8

sensing [33]. Due to the complex structure of biomolecules, elastic scattering from various parts of the cell may occur in all directions, thus producing serious background signals on SERS spectrum. Thus, improvement in identifying and screening the analytes efficiently in the microfluidic SERS chip is needed. An effective way is to optimize the SERS substrates or probes to improve the specificity and accuracy in analyte identification. By adding functional units on the SERS substrate, non-specific interference of other substances can be screened to exclude as far as possible [34]. Besides, the multilayered composite SERS probes can be

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constructed by wrapping the medium (such as silicon) outside in order to enhance the stability of the probe structure, thereby reducing the risk of signal interference in the

SERS detection, as SERS probes are often exposed to the environment. These methods will greatly improve the accuracy of the microfluidic SERS chip on the

SERS chip in clinical diagnosis and treatment.

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actual complex biological sample test and promote the application of microfluidic

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Microfluidic SERS chips have also been utilized for environmental monitoring. Microscale amount of contaminants in the environment may pose potential public

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health risks, so it is necessary to control any possible pollution and hazards in the environment. In particular, heavy metal irons are difficult to detect as they are

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commonly not SERS-active, but have serious impacts and harms on our living environment and health. By modifying the SERS probes or SERS-active substrates with aptamers, the target contaminating molecules can be specifically bound to the

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aptamer and thus, achieve the indirect SRES detection of heavy metal ions. Arsenic, as a ubiquitous pollutant in the environment, continuously affects people's health. Qi

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et al. [36] combined the microfluidic platform with SERS to implement the indirect detection of As(III) ions in a continuous flow. They conjugate glutathione (GSH) with 4-mercaptopyridine (4-MPY) modified on the silver nanoparticles substrates and probes. This is followed with a full mixture of the As(III) ions with the 4-MPY modified probes in a zigzag microfluidic channel. The original monodispersed probes would aggregate and Raman signals of 4-MPY were improved and in this way, the 9

As(III) ions could be detected. The research shows that this method can detect As(III) ions selectively as low as 0.67 ppb. Furthermore, aquatic pollution is also a major problem in environmental monitoring.

Zhou

et

al.

[37]

designed

a

compact

battery-controlled

nanostructure-assembled SERS system for capture and detection of small molecule pollutants in water. The microfluidic SERS system used an electrical heating constantan wire which is covered with the vertically aligned ZnO nanotapers decorated with Ag-nanoparticles, and then inserted it into a glass capillary (shown in

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Fig. 7). For SERS detection, the capillary is filled with a mixture of thermo-responsive microgels, Au-nanorods colloids and analyte solution as SERS substrate, and then heated up. The heating process creates high-density hotspots and as a result, strong SERS signals are detected. This integrated device can be used to

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measure methyl parathion in lake water, showing a great potential in detection of aquatic pollutants.

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Food safety has attracted an increasing attention in recent years. Food contamination is widespread in foods and agricultural products, including chemical

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hazards such as pesticides and melamine. and microbiological contaminants. Generally speaking, it is difficult to achieve rapid and on-site analysis of these

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contaminants. As the microfluidic chip has the advantages of small size and high integration, it has a good prospect in food contamination detection. By integrating SERS-active substrate into the microfluidic device, microfluidic SERS chips show

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enormous potential for application in the food industry, especially for contaminant detection in liquid foods [14]. However, due to the complicated processes of optical

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calibration and focusing during inspection, it is a great challenge to realize the portability. The development of optofluidic helps to solve this difficulty [38]. By the integration of SERS substrates with optical fibers, the laser is guided into the chips and the Raman signals is guided out. Yazdi and White [39] utilizes a porous microfluidic matrix formed by packed silica microspheres to concentrate silver nanoparticles as SERS substrate and adsorb analyte molecules, resulting in greatly 10

improved SERS detection performance (shown in Fig. 8). The optofluidic SERS device integrates two optical fibers to eliminate the difficulty of optical focusing and alignment during SERS detection. The result shows that the detection limits of two food contaminants, melamine and thiram, could down to 63 ppb and 50 ppt, respectively. In addition, portable diode lasers and handheld Raman spectrometers can also be used to fabricate the microfluidic system and then perform on-site SERS analysis. Kim et al. [40] utilizes an SERS-active gold nanofinger arrays as the substrate (Fig. 9) in a palm-sized portable Raman spectrometer to detect

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melamine in milk products. The portable Raman spectrometer is equipped with a diode laser for illumination over the sample surface. And the melamine sensing is

achieved by using an excitation laser. This portable device shows great sensitivity in the SERS test, and the experimental results can meet the requirements by the Food

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and Drug Administration (FDA). Furthermore, Shi et al. [41] for the first time

demonstrate the SERS chip for simultaneously quantifying Pb2+ and Hg2+ in real

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systems which could be potentially absorbed by human through food chains and drinking water. The chip is based on the combination of reproducible silicon

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nanohybrid substrates and can be well coupled with a hand-held Raman instrument for on-site detection (shown in Fig. 10). Results show that the SERS equipment can

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achieve dynamic ranges from 100 pM to 10 μM for Pb2+ and from 1 nM to 10 μM for Hg2+, with the detection limit down to 19.8 ppt for Pb2+ and 168 ppt for Hg2+. As more simple and convenient platforms have been developed [42], portable devices for

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on-site SERS detection are becoming gradually maturing. And the promising development suggests that the high-quality SERS chip is a powerful tool for on-site

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detection in the field of food safety. However, there are still challenges in the realization on the on-site analysis of

contaminants in real food matrix due to the complexity of the food matrix and the extremely low concentration of contaminants in food. More researches and experiments are needed regarding on the efficient and facile combination between microfluidic devices and SERS substrates, in order to improve the sensitivity of SERS 11

detection and accelerate the application of microfluidic SERS chips in food contaminants analysis in the future.

4. Challenges & Perspectives

Microfluidic SERS chip technology is a promising analytical tool and however has not yet broken through the laboratory science level. The research on the practical application in real sample analysis remains systematic investigation.

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Difficulties still exist in the preparation of the SERS-active substrates for microfluidic chips. The ordered and high enhance factors (EFs) fabrication of SERS-active substrates in microfluidic chips is difficult to be achieved, restricting the

realization of the reproducibility of SERS-microfluidic system. Compared to the

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colloid-based substrates, solid-state substrates can generate more consistent signals,

but there is still a long way to go in fabrication. Currently, the solid-state substrates

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manufactured by micro-electro-mechanical system (MEMS) fabrication technology has good order and uniformity, but requires expensive equipment, and cumbersome

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preparation process. Meanwhile, electron beam lithography (EBL) and focused ion beam (FIB) can accurately control the size and appearance of solid substrates and

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have been commonly used in preparation of solid-state SERS substrates [43]. But they face the same shortcomings that the equipment is very expensive and have difficulty in mass production for commercial use. Nevertheless, the preparation by chemical

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reduction has the advantages of simple equipment and low cost, but it is poor in controllability and order. Compared to the above two methods, SERS substrate

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prepared by electrochemical deposition method has the advantages of high order, good controllability and convenient operation, and has received extensive attentions [44,45]. But it requires further research on the adjusting of the composition, morphology and spacing of the nanostructures to improve the SERS substrate sensitivity. Promisingly, with the development of on-chip fabrication of SERS-active substrates, femtosecond laser direct writing technology has been proven to be a 12

powerful approach for the fabrication of photopolymer micro-nanostructures due to its unique merits of designability and reproducibility [46,47]. However, more in-depth research is still needed in terms of facile operation and precise control in order to improve the sensitivity of the microfluidic SERS systems. In addition, researches on microfluidic channels also contribute to the improvement of the microfluidic SERS chips’ reproducibility. The efficient mixer in microfluidic channel with concentrating functions need to be studied [14], which can help to improve the reproducibility and sensitivity of microfluidic system to a great extent.

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Furthermore, with the deepening research of the Raman technology, the SERS-active substrate exhibits the drawbacks of relatively weak selectivity in target

signals, which result in limited SERS application range. For example, when we examine the real food samples with microfluidic SERS chips, complexity of food

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matrices may lead to interference Raman signal spectral patterns because some food

debris share the similar chemical and physical properties to the target analyte. And

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this could eventually result in deviations in detection results. Hopefully, the problem could be overcome by decorating SERS substrates with specific antibodies or

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aptamers, which significantly increases the selectivity of the SERS-active substrates. Besides, the current SERS substrate composited of nanoparticles are difficult to be

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recycled and reused, making many portable microfluidic SERS devices very expensive for real market. Thus, how to expand the types of SERS-active medium for multi-functionality of the microfluidic SERS chips and achieve lower cost and easier

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implementation of SERS chips as much as possible while ensuring the perfect sensing and analysis functions has become a growing challenge for future’s research.

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Meanwhile, the low-cost microfluidic SERS chip has very important commercial and practical significance for biochemical testing in underdeveloped areas.

5. Conclusion

This paper briefly introduces the concepts and applications of microfluidic SERS 13

chips. By introducing SERS substrates into microfluidic technology to form microfluidic SERS chips, the existing optical detection methods can be effectively expanded and more in line with today’s testing needs. The existing achievements in preparing SERS-active substrate makes it possible for microfluidic SERS chips in biochemical testing like biosensing and food safety. We also give insights towards future developments. Although the microfluidic SERS chip has made remarkable progress, the higher level of the detection selectivity and reproducibility, the multi-functionality and integration of the chips are still the direction of future research.

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Finally, the paper presents the challenges in the preparation of microfluidic SERS substrates and provides suggestions for the development of future microfluidic SERS chips. It is foreseeable that SERS and microfluidic chip technology will be further

developed to promote perfect on-site and real-time detection technology, which will

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greatly benefit life and health science, biomedicine and others. The integrated,

automated SERS-LoC is bound to become a very important technology in the field of

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Acknowledgements

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sensing.

The work was supported financially by the National Natural Science Foundation of (No.

51802060),

the

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China

KQJSCX20170726104623185),

and

Shenzhen the

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KQTD20170809110344233).

14

Innovation

Shenzhen

Peacock

Project Group

(No. (No.

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(2010) 2902-2905.

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Figure List

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Fig. 1. Microfluidic SERS microsystem with integrated competitive displacement for DNA sequence detection, where silica microspheres functionalized with DNA

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probe−reporter pairs (inset) are packed against a frit. (Reprinted with permission from

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Yazdi et al. [23] Copyright 2012 American Chemical Society.)

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Fig. 2. (a) Schematic design of a gradient droplet microfluidic chip for SERS-based high throughput gradient analysis. Diluting the gold nanoflowers (AuNFs), injecting target analyte, buffer solution, and carrier oil into the channel inlets to form various concentrations of a target reagent trapped by the tiny droplets, and the SERS signals were measured using a He–Ne laser. (b) The system consists of two parallel layers: (i) The top layer is for target loading and serial dilution (ii) The middle layer is for sample reaction and SERS detection (c) Specific middle layer image: The droplet generation and SERS detection process is shown in image (left) and photo (right).

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(Jinhyeok Jeon et al. [25] Copyright The Royal Society of Chemistry 2019).

Fig.

Fig. 3. Scheme for laser fabrication of SMAs inside a microfluidic channel.

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(Bin-Bin Xu et al. [26] Copyright The Royal Society of Chemistry 2012).

Fig. 4. (a) Schematic of microfluidic chip, (b) The process of photoinduced growth of silver nanoaggregates and SERS measurements for CV in situ. (Adapted from Yan et al. [27] Copyright WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2017). 19

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Fig. 5. Procedure used to fabricate a 3D microfluidic SERS chip by

all-femtosecond-laser-processing (Adapted from Shi Bai et al. [30] Copyright 2018

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WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim).

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Fig. 6. Illustration of a single-cell encapsulation event within the microfluidic device. (A)The inset displays the PDMS Raman spectrum through a droplet without functionalized nanoprobes and the SERS spectra from wheat germ agglutinin (WGA)-functionalized nanoprobes. (B) Zoom-in of the cell membrane shows the expression of sialic acid. And the individual components of the nanoprobes are shown and named (Sialic acid expressed by cancerous prostate cells can be targeted using the lectin WGA [35].) (Adapted from Marjorie R. Willner et al. [32] Copyright 2018

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American Chemical Society).

Fig. 7. Schematic illustration of the battery-controlled composite SERS-based fluidic

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system. An electrical heating constantan wire covered with the ZnO nanotapers and Ag-nanoparticles is inserted into a glass capillary. The mixture in the capillary is heated up when SERS detection. (Reproduced from Zhou et al. [37] with permission from Nature Publishing Group).

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Fig. 8. Optofluidic SERS microsystem with packed microspheres for passive concentration, an integrated micromixer to promote adsorption of the target analyte,

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and integrated fiber optic cables for optical excitation and collection. (Adapted from

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Soroush et al. [39] Copyright 2012 American Chemical Society).

Fig. 9. Melamine sensing in milk products by using SERS sensor chips and a portable Raman spectrometer. (Adapted from A. Kim et al. [40] Copyright 2012 American 22

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Chemical Society).

Fig. 10. (a) Schematic of the portable silicon-based SERS analytical platform for

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on-site detection of Pb2+ and Hg2+ from industrial wastewater. (b) SERS spectra and (c) corresponding SERS relative intensities for Pb2+ and Hg2+ (Adapted from Yu Shi

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et al. [41] Copyright The Royal Society of Chemistry 2018).

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Table 1. Summary of application prospect of SERS-LoC Application

Range

of Cutting Edge Applications

Current status

SERS-LoC Single-cell analysis, small Trying to identify and Biomedical sensing

molecule or ion diagnosis screen the analytes in in people’s body, etc.

the microfluidic SERS chips efficiently.

Environmental monitoring

metal

detection,

ions Gradually

aquatic on-site and real-time

pollution monitoring, etc.

Food safety

examination.

contamination Sensitivity

and

on-site detection by using specificity

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Food

realizing

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Heavy

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portable SERS equipment

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SERS-active substrate needs to be improved.