Smartphone based visual and quantitative assays on upconversional paper sensor

Highlights

• The blue luminescence of NaYF₄:Yb/Tm upconversion nanoprobes would be quenched after additions of pesticide thiram through resonance energy transfer mechanism.

• The upconversional nanoprobes were fixed onto filter paper to form test paper for the rapid and visual detections.

• A tailor-made attachment was used for digitally imaging the luminescence variations on test paper and quantitative analyzing thiram.

• This work provides an initial demonstration of integrating upconversion nanosensors with smartphone digital imaging for point-of-care analysis on a paper-based platform.

Abstract

The integration of smartphone with paper sensors recently has been gain increasing attentions because of the achievement of quantitative and rapid analysis. However, smartphone based upconversional paper sensors have been restricted by the lack of effective methods to acquire luminescence signals on test paper. Herein, by the virtue of 3D printing technology, we exploited an auxiliary reusable device, which orderly assembled a 980 nm mini-laser, optical filter and mini-cavity together, for digitally imaging the luminescence variations on test paper and quantitative analyzing pesticide thiram by smartphone. In detail, copper ions decorated NaYF₄:Yb/Tm upconversion nanoparticles were fixed onto filter paper to form test paper, and the blue luminescence on it would be quenched after additions of thiram through luminescence resonance energy transfer mechanism. These variations could be monitored by the smartphone camera, and then the blue channel intensities of obtained colored images were calculated to quantify amounts of thiram through a self-written Android program installed on the smartphone, offering a reliable and accurate detection limit of 0.1 μM for the system. This work provides an initial demonstration of integrating upconversion nanosensors with smartphone digital imaging for point-of-care analysis on a paper-based platform.

Introduction

Paper based nanosensors appeared to be the most promising method for the low-cost, portable and rapid assay of trace substances in physiological fluids, agricultural products and many other fields to facilitate the applications in remote and resource-limited settings (Martinez et al., 2007, Martinez et al., 2008b). According to the detection targets ranging from metal ions to nucleic acid and enzymatic activity, various formats for paper sensors have been explored according to the variations of absorption or fluorescence signals (Parolo and Merkoci, 2013, Carrilho et al., 2009, Yetisen et al., 2013). For instance, Whiteside et al. have reported a patterned paper comprising hydrophilic paper bounded by hydrophobic polymer for the detections of glucose and protein in urine (Martinez et al., 2008a, Martinez et al., 2008b). We also have reported a photoluminescent graphene oxide paper sensor for the ultrasensitive visual assays of biomolecules through inkjet printing technology (Mei and Zhang, 2012). Nevertheless, these assay methods are inevitably interfered by background signals of complex samples. By the use of near-infrared laser as the excitation, upconversion nanoparticles (UCNs) effectively overcome this disadvantage, and have been widely explored in sensing and imaging chemo/bio targets (Chen et al., 2014, Gai et al., 2014, Zhou et al., 2015, Li et al., 2015a, Liu et al., 2011, Yuan et al., 2013). They are usually employed as energy donors to construct luminescence resonance energy transfer (LRET) mechanism based nanosensors by surface conjugating specific energy acceptors for selective assays of biomarkers (He and Liu, 2013), nuclear acid (Zhou et al., 2014), metal ions (Li and Wang, 2013) and so on (Arppe et al., 2015, Cen et al., 2014, Ma et al., 2014). After transferring onto paper substrates, upconversional sensors effectively avoided the strong absorption of excitation light by water molecules in solution or biological samples, and thus overwhelmingly minimized attenuation effects of luminescence intensities and detection limits of sensors (Zhong et al., 2014, Wen et al., 2013, Wang et al., 2013, Punjabi et al., 2014). Unfortunately, these paper sensors always are implemented with the naked-eye, which greatly suffer from the poor accuracy and precision.

Recently, the pervasive usage of smartphone and other consumer electronic devices provides a revolutionary opportunity for analytical community to directly readout optical signals on paper sensors (Wei et al., 2014, Lopez-Ruiz et al., 2014, Liang et al., 2014, Park et al., 2014, Noor and Krull, 2014, Noor and Krull, 2013). Smartphone and other consumer electronic devices exhibiting many attractive features including portability, accessibility, programmability, ease of use and low-cost can potentially be applied in the areas of environments monitoring, point-of-care testing and mobile diagnostics (Preechaburana et al., 2012, Preechaburana et al., 2014, Giavazzi et al., 2014, Ellerbee et al., 2009). Moreover, the use of CMOS photodiodes chip in smartphone camera as light sensors making them suitable to detect weak light signals, such as absorption and chemiluminescence, for the visual and rapid assays (Coskun et al., 2013, Oncescu et al., 2014). Roda et al. have reported that imaging and quantification of biochemiluminescence signals by a smartphone to detect total bile acids and cholesterol in serum or oral fluids (Roda et al., 2014). Despite the aforementioned progresses, no work has reported about the integration of upconversion nanosensors with smartphone-based detections probably due to the difficulty of effective acquisition of luminescent signals on test paper. Different from no use of excitation lights in previously reported works, smartphone based upconversion detection system needs a 980 nm laser as excitation light source as well as a dark cavity to alleviate image contrast caused by ambient light conditions, and methods for effectively excluding these peripheral lights from luminescence signals on test paper.

Herein, to visual and quantitative assays of pesticide thiram that is highly toxic if inhaled, and acute exposure in humans may cause headaches, dizziness, fatigue, nausea, diarrhea and other gastrointestinal complaints (Liu et al., 2012), we first synthesized a upconversional nanoprobe by conjugation of Cu²⁺ onto the surface of polymer-coated NaYF₄:Yb/Tm UCNs (as Scheme 1 A illustrated). Furthermore, by the virtue of 3D printing technology, we manufactured a smartphone attachment to orderly assemble low powered 980 nm laser, upconversional test paper and optical filter together, for rapid and accurate analysis of pesticide thiram. It can be seen from Scheme 1B, upconversional nanoprobes modified filter paper was placed into the mini-cavity of the optical accessory, and the blue luminescence on test paper could be efficiently quenched after additions of thiram. All of these luminescence intensities variations could be monitored by the smartphone camera. After the measurement, the smartphone could be detached from optical accessory and used for its original purpose.