Microfluidic diatomite analytical devices for illicit drug sensing with ppb-Level sensitivity

Highlights

• A microfluidic analytical device based on photonic crystal biosilica micro-channel array.

• Ultra-high sensitivity for illicit drug sensing down to 1–10 ppb in human plasma.

• Cost-effective platform for point-of-care applications.

Abstract

The escalating research interests in porous media microfluidics, such as microfluidic paper-based analytical devices, have fostered a new spectrum of biomedical devices for point-of-care (POC) diagnosis and biosensing. In this paper, we report microfluidic diatomite analytical devices (μDADs), which consist of highly porous photonic crystal biosilica channels, as an innovative lab-on-a-chip platform to detect illicit drugs. The μDADs in this work are fabricated by spin-coating and tape-stripping diatomaceous earth on regular glass slides with cross section of 400 × 30 μm². As the most unique feature, our μDADs can simultaneously perform on-chip chromatography to separate small molecules from complex biofluidic samples and acquire the surface-enhanced Raman scattering spectra of the target chemicals with high specificity. Owing to the ultra-small dimension of the diatomite microfluidic channels and the photonic crystal effect from the fossilized diatom frustules, we demonstrate unprecedented sensitivity down to part-per-billion (ppb) level when detecting pyrene (1ppb) from mixed sample with Raman dye and cocaine (10 ppb) from human plasma. This pioneering work proves the exclusive advantage of μDADs as emerging microfluidic devices for chemical and biomedical sensing, especially for POC drug screening.

Introduction

In recent years, the escalation of research interests in porous media microfluidics [[1], [2]], especially microfluidic paper-based analytical devices (μPADs) [[3], [4], [5]], have fostered a new spectrum of biomedical devices for point-of-care diagnosis and biosensing. μPADs can be fabricated by simple, low-cost processes using conventional photo- or soft lithographic techniques, utilizing either photoresists [6] or wax printing [7]. Advantages of using μPADs for microfluidic channels include: 1) ubiquitous and extremely cheap cellulosic materials; 2) capillary flow which enables fluid transport without using any external pump; and 3) compatible with many chemical and biomedical applications. Many different chemical and biological assays have been performed using μPADs, including for the detection of glucose [8], protein (albumin) [9], cholesterol [10], and heavy metals [11]. They have also been used as platforms for ELISA [12]. Especially, I. M. White’s group used inkjet-printed paper-based surface-enhanced Raman scattering (SERS) substrates for chromatographic separation and detection of target analytes from complex samples [13], which opened a new route for on-chip chemical sensing.

Other than μPADs, porous silica materials and devices also have attracted considerable attention for biosensing due to the use of their large surface area and pore volume to achieve high sensitivity [14,15]. The high porosity, which allows for the immobilization of target molecules not only on the external surface of the substrate but also inside of the pores, enables the loading of large amounts of sensing molecules, giving instant responses and high sensitivity. The optical transparency, on the other hand, permits optical detection through the bulk of the material. In addition, the surface groups and biocompatibility also makes porous silica one of the most potential materials for biosensing. Moon et al. have fabricated polymer and colloidal silica porous composite for nucleic acid biosensing [16]. Yang et al. have synthesized porous SiO₂ material and used it as enzyme immobilization carriers to fabricate glucose biosensors [17]. However, the pores in sol–gel derived silica lack a high degree of order, which results in random paths and consequently non-uniform diffusion of the analytes. A fraction of the sensing molecules might even be unreachable, leading to low response and poor spatial resolution [18].

Diatoms are unicellular, photosynthetic, bio-mineralized marine organisms that possess a biosilica shell, which is called the frustule. The two-dimensional (2-D) periodic pores on diatom surface enable it unique optical, physical, and chemical properties [19,20]. In recent decades, a variety of biosensors with ultra-high sensitivity using diatom biosilica have been reported [21]. Zhen et al. developed photoluminescence-based diatom biosensors that have been successfully applied for 2, 4, 6-trinitrotoluene (TNT) sensing [22]. De Stefano et al. have fabricated highly-selective biosensor for immuno-complex detection by modifying diatom frustules (Coscinodiscus concinnus) with antibodies [23]. From the optics perspective, the photonic crystal feature of diatoms could provide additional SERS enhancement when hybridized with plasmonic nanostructures [24,25]. Our group has developed an in-situ growth method for depositing silver nanoparticles (Ag NPs) on diatom for ultrasensitive, label-free TNT sensing [26,27]. Other than natural photonic crystal structures from living diatoms, diatomite consists of fossilized remains of ancient diatoms as geological deposits with billions of tons of reserve on earth. Therefore, diatomite is a type of naturally abundant photonic crystal biosilica, which has been widely used in industry as water filters, adsorbents, and medicine [[28], [29], [30]]. Diatomite has similar properties to diatoms such as highly porous structure, excellent adsorption capacity, and photonic crystal effects [31,32].

In this study, we report microfluidic diatomite analytical devices (μDADs), which consist of nano-porous photonic crystal biosilica channels for label-free biosensing of illicit drugs from complex biological samples using on-chip chromatography in conjunction with SERS sensing method. Previously, bio-inspired photonic crystals have been integrated into microfluidic systems as lab-on-a-chip system [33] and SERS has been employed for drug sensing [34]. In this research, Cocaine (C₁₇H₂₁NO₄) is chosen as the target analyte in our study, which is an alkaloid derived from coca leaves. Cocaine is one of the most widely used illicit drugs all over the world according to the latest World Drug Report from the United Nations Office on Drugs and Crime (UNODC). Cocaine is a potent stimulant of the central nervous system that leads to a state of increased alertness and euphoria. Its effect is similar to that of amphetamines but with shorter duration. In this study, we report using μDADs for on-chip chromatography-SERS to separate and detect cocaine from real biofluidic samples. The μDADs achieve nearly 1000 times better limit of detection (LOD) than normal chromatography plates to 1–10 ppb level, which is comparable or even higher than that of many laboratory analysis techniques [35], which will be discussed in Section 3.6.