Quantitative remote and on-site Hg²⁺ detection using the handheld smartphone based optical fiber fluorescence sensor (SOFFS)

Highlights

• Smartphone based optical fiber fluorescence sensor (SOFFS) is designed for remote and on-site Hg²⁺ detection.

• SOFFS is handheld with inner power supply, and fiber probe can be easily replaced, supporting remote and on-site detection.

• Highly specific detection of Hg²⁺ with a linear relation between 1 nM and 1 μM with a detection limit of 1 nM is reached.

• Simple and rapid operations within 3 min are required for single Hg²⁺ detection.

Abstract

In order to detect Hg²⁺ in remote and on-site mode, we design a smartphone based optical fiber fluorescence sensor (SOFFS), which is composed of a semiconductor laser for fluorescence signal excitation, a quantum dot modified fiber probe for Hg²⁺ sensing, a smartphone with a filter for fluorescence signal collection, and a fiber coupler for connecting fiber probe, laser and smartphone. With the evanescent wave on the optimized combination tapered fiber probe surface, SOFFS has an ultra-low detection limit of 1 nM and a wide detection range between 1 nM to 1000 nM. Besides, it is handheld with a small size of 79×57×154 mm³ and inner power supply, and the combination tapered fiber probe can be easily replaced, supporting the remote and on-site applications. Additionally, a smartphone application is designed for automatically quantifying the fluorescence signals for accurate and rapid Hg²⁺ detection. Since SOFFS is compact and cost-effective, moreover, it can measure Hg²⁺ in fast speed, excellent selectivity, high accuracy and sensitivity, it is a potential tool for remote and on-site applications.

Introduction

As one of the most toxic and ubiquitous heavy metal pollutants, mercury is seriously harmful to human health and environmental safety [1,2]. Mercury exists in various forms such as metal oxide, metal ion and organic complexes. Among them, Hg²⁺ is the most stable form [3], and even trace Hg²⁺ in water could threaten human health [4], thus developing tools for sensitive and accurate Hg²⁺ detection is required. Methods such as atomic absorption spectrometry [5], gas chromatography [6], high performance liquid chromatography [7] and electrochemistry [8] have been widely used for Hg²⁺ detection. Though they are able to measure the Hg²⁺ concentration precisely, they not only rely on time-consuming and complicated operations, but also require huge and expensive experimental setups, limiting their on-site applications.

In order to develop on-site Hg²⁺ detecting tools, various point-of-care testing (PoCT) sensors [[9], [10], [11]], especially smartphone based fluorescence sensors [[12], [13], [14], [15], [16]] have been proposed due to their excellent performances with the smartphone camera for high quality signal collection and the smartphone processor for rapid data analysis. Chen et al. presented a paper-based analytical device using colorimetric gold nanoparticles for Hg²⁺ detection [17]. Xiao et al. designed a portable smartphone readout device combining with the aptamer-assay nanosensor [18]. Motalebizadeh et al. reported smartphone-based PDMS microfluidic kit [19]. Wei et al. proposed gold nanoparticle based colorimetric assay relying on the smartphone readout device [20]. Shan et al. designed a miniaturized smartphone fluorescence microscope for Hg²⁺ detection using the T-Hg²⁺-T sensing tactic [21]. These PoCT instruments are small in size, low in cost, and can be easily used for on-site inspection, but most of them still require complicated sample preparation; moreover, they cannot realize remote detection, which is of significance for hazardous environment detection.

Optical fiber probes, which have the advantages of simple operations and excellent robustness, can be potentially adopted in remote and on-site Hg²⁺ detection [22,23]. Gonçalves et al. reported an optical fiber fluorescence sensor based on carbon dots for Hg²⁺ detection [24]. Long et al. combined evanescent wave optical fiber fluorescence sensor with aptamer-based biosensor to detect Hg²⁺ [25]. However, most of these optical fiber sensors rely on bulky sources and expensive fiber spectro-photometers or detectors, still restricting their on-site applications.

In order to detect Hg²⁺ in remote and on-site mode, here we design a simple, economical and handheld smartphone based optical fiber fluorescence sensor (SOFFS) as shown in Fig. 1. In SOFFS, a semiconductor laser is a source of the excitation light, connecting to the quantum dot (QD) modified fiber probe through a fiber coupler. The fiber probe is optimized as combination tapered configuration to reduce the transmission leakage loss between the sensing and non-sensing regions. The CdSe/ZnS QDs are used for providing fluorescence signals due to their broad excitation spectra, tunable narrow emission spectra, high quantum yields and excellent chemical stability [[26], [27], [28], [29], [30]]. Due to electron transfer [[31], [32], [33]] and chelation recombination [[34], [35], [36]], the fluorescence intensity of QDs is quenched in Hg²⁺ environment. The quenched fluorescence signals are collected to the smartphone camera with a filter still from the fiber probe through the fiber coupler. According to the quantification on the fluorescence quenching, the Hg²⁺ concentrations could be accurately measured. SOFFS does not require separate optical components such as prisms, slides or optical benches, avoiding the precise optical alignment. All the optical components are integrated with the smartphone in a 3-D printing shell with a small size of 79 × 57 × 154 mm³, and the QD modified fiber probe can be installed and replaced conveniently through the fiber flange on the side of SOFFS, which made the system simple and portability. A smartphone application is also designed for automatically quantifying the fluorescence quenching for accurate Hg²⁺ detection. Furthermore, for single measurements, only 3 min is required including fiber probe replacement, fluorescence image recording and data analysis. Since SOFFS can easily realize remote detection, rapid monitoring, and field measurement, it can be used for Hg²⁺ detection in remote and on-site applications.