| Lead ion (Pb2+) is a ubiquitous metallic pollutant in the environment and had been used recklessly until very recently. Nowadays, lead pollution has become a persisting problem and caused a long-lasting danger to human health. Even very low level of lead exposure can correspondingly result in high blood pressure, neurological, cardiovascular and developmental disorders, which lead to particularly serious problems in children including slowed motor responses, IQ decreasing and hypertension. The Centers for Disease Control and Prevention （CDC） has defined that the whole blood lead concentration>0.48 μM （100 μg/L） in children as indicative of significant exposure and recommends chelation therapy. Due to the toxicity of Pb2+, various equipment built up for the quantification of Pb2+ in the past decades, such as inductively coupled plasma mass spectrometry, atomic fluorescence spectrometry and atomic absorption spectroscopy, et al. Although these strategies provide accurate, ultrasensitive detection, most of them require tedious sample pretreatment before analysis, and generally are not amenable to be handled by unskilled personnel. Therefore, it is highly desirable to develop a portable, easy use, on-site, selective and sensitive method for Pb2+ assaying.Compared with those sophisticated equipment, colorimetric sensors gained increasing attention for its very short assay time, relatively low cost and no requirement for skillful technicians. Until now, considerable efforts have been devoted to the assay of Pb2+ utilizing different receptors, including DNAzymes, proteins, oligonucleotides, polymers and nanomaterials. There are several literature reports on colorimetric sensors to detect Pb2+ in aqueous solution. However, low shelf stability of probes, poor portability and matrix interference have resulted in limited number of applications. In order to overcome the limitations of gold colloids and achieve to widely used, the strategy of assembling Au NPs on solid substrates, such as quartz, ITO glass, sol-gel matrix, et al., have been used. Electrospun nanofibers, which are at the forefront of advanced fibrous materials, combine the robust mechanical strength, low density, fine flexibility, extremely high aspect ratio and ease of scalable synthesis from various materials （polymer, ceramic, metal, carbon and so on）. These fibers hold great promise as an exceptional material for constructing colorimetric Pb2+ sensing membranes. Here, we reported the fabrication of colorimetric strips for visual lead ion recognition utilizing functional electrospun nanofibrous membranes. The approach is based on the hierarchically structured nanofbers, which are created by the combination of electrospinning and subsequent surface modification. Key to our successful design is that the strips enable reversible, selective and sensitive recognition of Pb2+ down to 0.2 μM with rapid sensing responses, showing promising and potential applications for preliminary screening of lead poisoning. The detailed contents are summarized below:（1） The sensor design is realized utilizing electrospun Nylon-6/Polyvinylidene fluoride （N6/PVDF） nano-fibers/nets （NFN） membranes assemble L-glutathione conjugated Au nanoparticle probes （Au@GSH）, which can aggregate to trigger a vivid color change of pink to purple upon incubation with Pb2+. The strips exhibited excellent stability, rapid response （10 min）, wide detection range （10 to 500 μg/dl） and low naked-eye detection limit of 10 μg/dl. Furthermore, the NFN-based strip displayed excellent selectivity to other possible metal cations.（2） A facile, ultrasensitive and selective sensor strip utilizing electrospun polyamide-6/nitrocellulose （PA6/NC） nano-fibers/nets （NFN） membranes assemble bovine serum albumin decorated Au nanoparticles （BAu probe） for naked-eye colorimetric assay of Pb2+ has successfully prepared through dual-component alternate distribution multifluidic electrospinning technique. Upon exposure to a series of metal ions, only Pb2+ could induce a pink-to-white color change, with a low detection limit of 0.2 mM by naked eye. Additionally, the colorimetric responses are represented in visualized quantitative by calculated color difference from L*a*b* values.（3） A novel colorimetric strip that relies on a polydiacetylene （PDA） embedded polyacrylonitrile nanofibrous membranes （PAN NFM） was successfully fabircated. The Pb2+-chromic PDA is prepared from controlled mixtures of 10, 12-Pentacosadiynoic acid （PCD A） and PCDA-5EG, a PCD A derivative with a pentaethylene glycol headgroup. Upon exposure to a series of metal ions, only Pb2+ could induce a color change, and the strip with a naked eye detection limit of 0.48 μM undergo a brilliant color transition, from blue-to-red, in a concentration-dependent manner and all the color transitions of strips are quantitatively visualized by employing a chromatic framework.（4） A novel colorimetric strip based on a SiO2 nanoparticle （NPs） decorated polydiacetylene embedded polyacrylonitrile nanofibrous membranes （PAN NFM） was prepared, which undergoes a brilliant blue-to-red color transition as well as Turns-On’ fluorescence upon incubation with Pb2+. The SiO2 NPs are certified as an efficient sensitivity promoter when their content reaches 0.5 wt% in the strip. With this understanding, the color change that is caused by 0.24 μM Pb2+ at ambient temperature can be easily perceived by naked eyes. Furthermore, RGB （red-green-blue） digital parameters obtained from photographs of the strips and automatically read out via a smartphone were processed statistically through principal component analysis.（5） By virtue of the affinity of pyromellitic dianhydride （PMDA） with lead（Ⅱ） ion （Pb2+） and inherent structure merits of electrospun nanofibrous membrane, a novel solid-phase nanofibrous material was facilely fabricated by modification deacerylated cellulose acetate membrane with PMDA （DCA-PMDA）. The resultant DCA-PMDA can be applied for naked eye detection and removal of Pb2+ from water matrixes simultaneously by a simple filtration and follow with Na2S solution treatment. Importantly, the membranes can be applied for colorimetric detection of Pb2+ with a naked eye detection limit of 0.048 μM. Under the same circumstance, the maximum adsorption capacity was determined to be as high as 326.80 mg/g. |
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