WEARABLE DESIGN FOR OCCUPATIONAL SAFETY OF PB 2 + WATER POLLUTION MONITORING BASED ON FLUORESCENT CDS

: The indiscriminate discharge of industrial and domestic wastewater leads to the pollution of production, living, and landscape waters with heavy metals, including Pb 2 + . To protect people working in environments with risk of water pollution by Pb 2 + , the Pb 2 + chemosensor based on ﬂ uorescent carbon dots ( CDs ) was designed in this study. Based on quenching of the ﬂ uorescence of CDs via electron or energy transfer between Pb 2 + and CDs, the sensor induces a signi ﬁ cant “ dark blue to light blue ” ﬂ uorescence burst color change under the naked eye. Results suggest that the ﬂ uorescence intensity of CDs positively correlated with the concentrations of Pb 2 + ( R 2 = 0.823 – 0.986 ) , which is conducive to the detection of Pb 2 + - containing wastewater by CDs. On this basis, we integrated CD ﬂ uorescent sensors into wearable gloves via a mediated coating method. With no additional burden ( off - site, long lead times, high cost, etc. ) , the designed wearable gloves enable front loading of the detection window for Pb 2 + - contaminated water to protect the wearer from poisoning. Eight watershed environmental occupants reported a 57.42% reduction in occupational anxiety after using our safety gloves. We believe that the proposed ﬂ exible and stable wearable sensing system will not only have great potential applications in Pb 2 + ﬁ eld detection but also stimulate the development of other environmental pollution sensing devices.


Introduction
The rapid development of socio-economic and industrialization have caused the pollution of the water environment with pesticide residues, heavy metals, and radioactivity, among which heavy metal pollution is of great concern.Among the many sources of heavy metal pollution, Pb 2+ is considered to be one of the most hazardous metals.Pb is located in the IVA group of the sixth cycle of the periodic table of chemical elements and is one of the most abundant toxic heavy metals.Pb 2+ is widely used in many fields such as lead glass, photovoltaic cells, paints, pigments, chemicals, and lead-acid batteries.Low levels of Pb 2+ exposure can cause many adverse health effects.Pb 2+ can cause abnormalities in the brain, central nervous system, and intelligence [1,2], which will make it dangerous to work in the environment containing Pb 2+ wastewater.The research and establishment of rapid detection methods for Pb 2+ are of great significance for the prevention and control of water pollution and occupational safety hazards caused by Pb 2+ .
A large number of powerful electrochemical [3][4][5], colorimetric [6][7][8], and fluorescent-based [9][10][11][12] Pb 2+ monitoring sensors have been developed and applied in the last few decades.Huang et al. reported that the synthesized high-fluorescent gold nanoclusters using protamine as both a reducing and a stabilizing reagent were successfully used as a nanoprobe for the fluorescence detection of Pb 2+ [9].Moreover, MoS 2 was synthesized as a nanoprobe by the hydrothermal method to design the ion sensors of Co 2+ , Cd 2+ , and Pb 2+ for the detection of actual water samples [10].Although scholars in this field have made these achievements, most of the studies mainly focused on the preparation and detection of sensors in solution form.Solid-state sensors with good portability and stability are more attractive in practical occupational safety application scenarios.Among the various solid-state sensing materials, fluorescent carbon dots (CDs) have received increasing attention in recent years.CDs as new fluorescent carbon nanomaterials with a size of less than 10.0 nm are rich in carboxyl and amino groups on the surface [13].Since 2004, CDs have been widely used in heavy metal pollution tracking, optical catalysis [14,15], biochemical sensing [16], and drug delivery [17,18] due to their high stability, low toxicity, high water solubility, and good biocompatibility [14].Recently, the application of CDs has attracted considerable attention in heavy metal pollution tracking, which has not been uncommon for international scholars to study in this direction.Wen et al. prepared multicolor fluorescent CDs with a quantum yield of 24.1% from pigskin and successfully used them for Co 2+ ion detection and multicolor cellular imaging [15].Zong et al. prepared CDs using mesoporous silica spheres as nanoreactors, which were successfully applied as fluorescent probes for the highly sensitive and selective detection of Cu 2+ and L-cysteine [19].Chen et al. used citric acid monohydrate and ethylenediamine as raw materials to prepare CDs for monitoring and imaging of Cu 2+ and S 2− in living cells [20].Iqbal et al. prepared blue fluorescent CDs from anhydrous citric acid and 1,10-o-phenanthroline, which were successfully applied to the determination of trace Fe in real samples of milk [21].
Few studies have been conducted to prepare chemical sensors using CDs that can detect Pb 2+ -contaminated water samples flexibly and efficiently.However, the pollution of waters affected by Pb 2+ has become an occupational safety issue of worldwide concern.The water-soluble CDs were prepared by hydrothermal synthesis using typical water pollutant cyanobacteria as a raw material.Based on the burst effect of Pb 2+ on the fluorescence of CDs, a new method was established for the rapid detection of Pb 2+ with simple operation, low cost, and high sensitivity.Then, the obtained CD fluorescent sensor is applied to occupational safety wearable for the field detection of Pb 2+ in actual water samples.

Materials
The CD source was derived from cyanobacteria, which salvaged from Taihu Lake (Wuxi, China).Dialysis membranes (2,000 Da; Spectrum labs) were purchased from Sigma-Aldrich (Shanghai, China).Pb(NO 3 ) 2 was supplied by Sinopharm Chemical Reagent Co., Ltd.

Synthesis of CDs
The procedure for the synthesis and preparation of CDs is depicted in Figure 1.In detail, the raw material was first air-dried for 48 h, oven-dried at 60°C for 48 h, and passed through a 70 mesh.The CDs were synthesized via the hydrothermal method using the obtained powder as the carbon source [22].The cyanobacteria powder (20 g) was treated in 300 mL of purified water under stirring at room temperature for 15 min.Then the mixture was transferred into a polytetrafluoroethylene-equipped stainless-steel autoclave (500 mL) and heated at 200°C for 8 h.After cooling, the mixture was centrifuged at 6,000 rpm for 20 min, the supernatant was taken for dialysis (2,000 Da) for 48 h to obtain the aqueous solution of carbon quantum dots, and then freeze-dried to obtain solid carbon quantum dots.

Characterization of CDs
The size of the obtained CDs was obtained using a high-resolution transmission electron microscope (JEOL JEM 2100F, Japan).Photoluminescence analysis was measured with an F-7000 fluorescence spectrophotometer (Hitachi, Japan).

Wearable glove design
In order to apply the synthesized fluorescent CDs to practical occupational safety scenarios, we further used them in the preparation of wearable gloves.As a carrier for fluorescent CD chemosensors, the designed wearable glove achieves the anteriority of Pb 2+ contamination detection without adding additional burden to the occupants.At the same time, wearable gloves are a safety barrier for occupational personnel to avoid physical exposure to contaminated water.CDs and polydopamine (PDA) mediators are homogeneously mixed together by ultrasound [23].Meanwhile, we prepared gloves containing polyvinyl acetate (PVA) material on the index fingertips.Briefly, CDs (10 mg) were added into the PDA (10 mL, 1.0 mg/mL) solution.Sequentially, the obtained solution was contacted with the gloves containing PVA material for 30 min at 25°C.Ultimately, the obtained gloves were kept at 40°C in an oven for 10 min.The PDA mediator mixed with CDs is co-soluble with the PVA polymer on the index finger tip of the glove by coating to form a stable layer.

Characterization of CDs
The size and fluorescent properties of CDs are depicted in Figure 2. It appears that CDs are well separated with a diameter ranging from 1.5 to 4.2 nm.As depicted in Figure 2b, the fluorescence properties of the as-prepared CDs were investigated.Fluorescence spectra displayed that the optimal emission wavelength of CDs appeared to be 470 nm via 380 nm excitation, which further states clearly blue fluorescence of cyanobacteria-derived CDs.Noticeably, as the excitation wavelength increased from 300 to 400 nm, the fluorescence intensity of CDs also changed correspondingly.Specifically, when the excitation wavelength is less than 380 nm, the fluorescence intensity of CDs gradually increased, and then decreased with

Spectral investigations in the presence of Pb 2+
We explored the feasibility of using such CDs (e.g., fluorescence intensity) for Pb 2+ detection, and the changes in fluorescence intensity of CDs are shown in Figure 3.As can be seen in Figure 3a, the CD solution without Pb 2+ exhibited a strong fluorescence intensity at 470 nm.By contrast, the presence of Pb 2+ resulted in a significant decrease in the fluorescence intensity, suggesting that Pb 2+ could effectively quench the fluorescence of CDs. Figure 3a demonstrates that fluorescence intensity of CDs at 470 nm gradually decreases with an increased Pb 2+ concentration, indicating that the sensing system is sensitive to Pb 2+ concentration.This phenomenon evidenced that Pb 2+ can be attributed to the possible quenching of the fluorescence of CDs via electron or energy transfer [29,30].It is clear that the fluorescence intensity of CDs positively correlated with the concentrations of Pb 2+ (R 2 in the range of 0.823-0.986),demonstrating that Pb 2+ played a dominating role in the fluorescence intensity of CDs in the present study (Figure 3b).However, for low ( ＜8 μm (μmol/L)/high (8-56 μm)) concentrations of Pb-contaminated wastewater, the CD detection performance works best (R 2 in the range of 0.936-0.986).

Design and testing of wearable gloves
The mediated coating method was used to obtain wearable gloves, and the specific structure and detection process of the gloves are shown in Figure 4.The tip of the index finger of the wearable glove, the detection area (containing CDs), is off-white at room temperature and emits a uniform dark blue fluorescence when illuminated using a 365 nm UV lamp.When the detection area of the gloves was immersed in water samples containing different Pb 2+ concentrations, which showed distinctly different degrees of fluorescence burst effect.When the Pb 2+ concentration increased from 0 to 56 μm in the water sample, the detection area of the glove showed a gradual and continuous color change of the fluorescence burst, as shown in Figure 5a.To show the process of naked-eye detection more visually, the color change was marked trajectory in the standard chromaticity coordinate model proposed by International   Commission on Illumination (CIE), as shown in Figure 5b.The starting and ending points of the trajectory are the color coordinates of the glove monitoring area when the Pb 2+ concentration is 0 and 56 μm.
In addition, the feasibility of using gloves was investigated to detect Pb 2+ in actual water samples.The water from Taihu Lake, Wuxi, China and the seawater from Beilun Port, Ningbo, China were filtered to remove insoluble substances.The two pretreated actual water samples were divided into seven portions and 0, 9.00, 18.00, 27.00, 36.00,45.00, and 56.00 μm of Pb 2+ were added into water samples, respectively.It was tested that the fluorescence color change pattern of the wearable gloves was almost the same between the actual water sample and the pure water sample for the same concentration of Pb 2+ , as shown in Figure 5a.This demonstrates the suitability of our wearable gloves for visual detection of Pb 2+ in real water samples without the need for complex pretreatment processes.The designed wearable gloves are highly practical for real-time Pb 2+ detection in occupational scenario scenes.In addition, the prepared gloves can be put on and taken off and operated professionally as easily as untreated gloves.Their functionalization did not place an additional burden on the wearer.The fluorescence intensity of the glove test area remained essentially unchanged for 1 month, showing good stability in actual use and storage.When the fluorescence is quenched by Pb 2+ in contaminated water, the gloves cannot be recovered and reused.Considering the value of gloves in practical application scenarios and the possible cost in mass production, it is considered acceptable as a disposable product after reaction triggering.Objective tests show that the applicability to real water samples and stability of gloves have reached high practical application requirements.

Subjective test of career anxiety
Working in occupational environments with safety risks for long time can cause anxiety.Occupational anxiety can have a serious negative impact on employees' health and productivity.The wearable gloves are designed to reduce safety risks by enhancing the active hazard awareness of professionals through the front of the Pb 2+ water contamination detection window.This promises to be an effective way to alleviate employees' feelings of occupational anxiety.
To explore and enlighten the positive value of similar occupational safety wearables on users' occupational anxiety and mental health, we invited eight people engaged in landscape water cleaning, aquatic salvage, and domestic wastewater treatment to conduct a subjective test of occupational anxiety.Considering the uncertainty and inconsistency in the knowledge status of Pb 2+ water pollution among the eight participants, we first provided them with relevant scientific knowledge.Then, the participants had a complete understanding of the used method, working mechanism, and testing effect of the wearable gloves, and experienced the use of the wearable glove design prototype.Finally, participants used a 0-100% scale to record their feelings of occupational anxiety while working with and without our wearable gloves: 0% means no anxiety at all and 100% means the anxiety is at their tolerance limit.The results of the test are shown in Table 1.Eight participants experienced a 57.42% reduction in occupational anxiety when using the wearable gloves compared to when they were not using them.The subjective test proves that the designed gloves can effectively help the water workers improve their occupational safety and reduce their anxiety.Even if there are no target pollutants that harm workers in the occupational environment, the smart wearable with powerful functions and low additional burden can also provide effective practical help to the risk occupational workers in terms of occupational mental health.

Conclusions
We designed and prepared a novel CD fluorescent Pb 2+ chemical sensor by hydrothermal synthesis using local water pollutant, cyanobacteria from Taihu Lake.The design of the sensor relies on a fluorescence burst chemical reaction between the fluorescent CDs and Pb 2+ to induce a "dark blue to light blue" color change visible to the naked eye under UV light.We innovated the designed sensor to wearable sensing gloves for water occupational safety scenarios.The wearable gloves we have designed are portable, highly sensitive, and easily and effectively detect Pb 2+ even in complex actual water use.The gloves can be used in water work scenarios where there is a risk of Pb 2+ contamination, such as landscaped waters requiring maintenance cleaning, domestic wastewater areas requiring treatment, and production wastewater areas requiring salvage.Professionals can quickly and directly identify Pb 2+ -contaminated water samples with wearable sensing gloves.It is notable that this wearable glove with an integrated fluorescent CD sensor not only enhances the safety of occupational personnel through the front contaminated water sample detection window but also shows potential in reducing occupational anxiety in tests.In the objective and subjective tests, the designed gloves show high theoretical value and practical operability in the face of water pollution occupational safety and occupational mental health problems.We hope that this study can inspire the development of more robust and low additional burden detection devices in other integrated pollution environments.

Figure 1 .
Figure 1.Schematic diagram of CD synthesis and preparation procedures.

Figure 2 .
Figure 2. (a) TEM images of CDs and (b) fluorescence spectra of CDs.

Figure 3 .
Figure 3. (a) The fluorescence spectra of CDs with Pb 2+ in the emission wavelength of 470 nm and (b) the fluorescence intensity of CDs in response to Pb 2+ (0.0-56 μm).

Figure 4 .
Figure 4. Schematic diagram of the structure and detection process of wearable gloves.

Figure 5 .
Figure 5. (a) Effect of visual inspection of pure water, lake water, and seawater by gloves and (b) CIE chromatogram of the burst effect of glove on aqueous Pb 2+ solutions of 0-56 μm.

Table 1 .
Results of subjective tests of occupational anxiety