Quantitative remote and on-site Hg2+ detection using the handheld smartphone based optical fiber fluorescence sensor (SOFFS)
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.
Section snippets
Chemicals and reagents
3-Mercaptopropionic acid (MPA) capped carboxyl (COOH) coated CdSe/ZnS QDs with the emission spectrum of 620 ± 15 nm and the average diameter of ∼20 nm were purchased from Mesolight (China). (3-Aminopropyl)triethoxysilane (APTES, 98%), N-(3-Dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride (EDC), N-Hydroxysuccinimide (NHS) and bovine serum albumin (BSA) were purchased from Aladdin (China). Other chemicals were of analysis grade from local sources. All the reagents were used without any
Optimization, fabrication and verification of QD modified combination tapered probe
The structure of the combination tapered fiber probe for Hg2+ sensing is shown in Fig. 5(A), which is composed of a tapered portion and a cylindrical portion. The tapered portion functions as the transition section while the cylindrical portion is used for sensing. Since the cladding in non-sensing region is replaced by the aqueous phase in sensing region, the mode capacity between the non-sensing portion and the sensing portion is different [39]. The mode capacity can be calculated using V
Conclusion
To be concluded, we design SOFFS for remote and on-site Hg2+ detection. The optimized combination tapered fiber probe in SOFFS can detect Hg2+ in remote mode with an extremely low detection limit of 1 nM and a wide detection range from 1 nM to 1000 nM. The fiber probes have high fabrication quality, such as excellent fabrication uniformity and long preservation period of 3 months, and it is proved that the fiber probes have good selectivity and high accuracy in Hg2+ sensing. Additionally, SOFFS
CRediT authorship contribution statement
Ting Liu: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Supervision, Validation, Writing - original draft, Writing - review & editing. Wenqi Wang: Conceptualization, Data curation, Formal analysis, Funding acquisition, Investigation, Methodology, Validation, Writing - original draft, Writing - review & editing. Dan Jian: Resources, Investigation, Methodology, Software, Writing - original draft. Jiahao Li: Resources, Investigation,
Declaration of Competing Interest
F.L. and S.W. are co-founders of Sinmotec LLC which commercializes single molecule sensing and imaging tools.
Acknowledgements
The work was supported by the National Natural Science Foundation of China (Grant no. 61505057, 31870154, 61705092); the Promotion Program for Young and Middle-aged Teacher in Science and Technology Research of Huaqiao University (Grant no. ZQN-PY603); and the Natural Science Foundation of Jiangsu Province of China (Grant no. BK20170194, BE2018709).
Ting Liu received her bachelor degree in optical information science and technology from Nanjing University of Science and Technology (Jiangsu, China) in 2009, and Ph.D. degree in optical engineering from Tsinghua University (Beijing, China) in 2014. Currently, she is an assistant professor at College of Mechanical Engineering and Automation, Huaqiao University (Fujian, China). Her research interests include optical fiber biosensor and nanotechnology.
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Ting Liu received her bachelor degree in optical information science and technology from Nanjing University of Science and Technology (Jiangsu, China) in 2009, and Ph.D. degree in optical engineering from Tsinghua University (Beijing, China) in 2014. Currently, she is an assistant professor at College of Mechanical Engineering and Automation, Huaqiao University (Fujian, China). Her research interests include optical fiber biosensor and nanotechnology.
Wenqi Wang is a master student at College of Mechanical Engineering and Automation of Huaqiao University. She received her bachelor degree from Huaqiao University in 2016. She is researching on optical fiber fluorescence sensing techniques.
Dan Jian is a master student in Computational Optics Laboratory, Jiangnan University (Jiangsu, China). She received her bachelor degree from Jiangnan University. She is researching on smart optical sensing and imaging techniques.
Jiahao Li is a master student at College of Veterinary Medicine, Nanjing Agricultural University (Jiangsu, China). He received his bachelor degree from Anhui Science and Technology University. He is researching on virus detection using single molecule techniques.
He Ding is a master student at College of Mechanical Engineering and Automation of Huaqiao University. He received his bachelor degree from Shangdong University of Technology in 2018. He is researching on optical fiber biosensor techniques.
Dingrong Yi received her Ph.D. degree in intelligent instrument in 2002 from McGill University (Canada). From 2002–2011, she was a Research Fellow at University of Toronto (Canada). Currently she is a professor at College of Mechanical Engineering and Automation, Huaqiao University. Her research interests are multispectral imaging, 3-D machine vision, high resolution microscopic imaging and biosensors.
Fei Liu received her Ph.D. degree in Biophysics at Case Western Reserve University (CWRU) in USA in 2010. From 2010–2012, she was a post-doctoral fellow at University of California Berkeley. In 2012, she returned to China, and has become a professor at College of Veterinary Medicine at Nanjing Agricultural University since then. Dr. Liu’s group is an interdisciplinary lab working at the interface of biology, physics, chemistry, molecular biology and nanotechnology.
Shouyu Wang received his Ph.D. in optical engineering from Nanjing University of Science and Technology in 2015. Currently, he is an associate professor and PI at Computational Optics Laboratory, Jiangnan University. He is also the adjunct research fellow at the Single Molecule Nanometry Laboratory, Nanjing Agricultural University. His main interests include optical imaging and sensing.