NanoPOP: Solution-Processable Fluorescent Porous Organic Polymer

Jun 18, 2019 - Spot tests using this test paper as the detection device were carried out. ... These shifts are typical for Hg2+ binding,(57) and consi...
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NanoPOP: Solution-Processable Fluorescent Porous Organic Polymer for Highly Sensitive, Selective, and Fast Naked Eye Detection of Mercury Yankai Li,†,‡ Yulong He,† Fangyuan Guo,† Shenping Zhang,† Yanyao Liu,‡ William P. Lustig,‡ Shiming Bi,† Lawrence J. Williams,‡ Jun Hu,*,† and Jing Li*,‡

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School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China ‡ Department of Chemistry and Chemical Biology, Rutgers University, 123 Bevier Road, Piscataway, New Jersey 08854, United States S Supporting Information *

ABSTRACT: Fluorescence-based detection is one of the most efficient and cost-effective methods for detecting hazardous, aqueous Hg2+. We designed a fluorescent porous organic polymer (TPA-POPTSC), with a “fluorophore” backbone and a thiosemicarbazide “receptor” for Hg2+-targeted sensing. Nanometer-sized TPA-POPTSC spheres (nanoPOP) were synthesized under mini-emulsion conditions and showed excellent solution processability and dispersity in aqueous solution. The nanoPOP sensor exhibits exceptional sensitivity (Ksv = 1.01 × 106 M−1) and outstanding selectivity for Hg2+ over other ions with rapid response and full recyclability. Furthermore, the nanoPOP material can be easily coated onto a paper substrate to afford naked eye-based Hg2+detecting test strips that are convenient, inexpensive, fast, highly sensitive, and reusable. Our design takes advantage of the efficient and selective capture of Hg2+ by thiosemicarbazides (binding energy = −29.84 kJ mol−1), which facilitates electron transfer from fluorophore to bound receptor, quenching the sensor’s fluorescence. KEYWORDS: nanoPOP, porous organic polymers, binding energy, self-consistent framework

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INTRODUCTION Mercury-ion (Hg2+) contamination in water has become a serious global issue due to its growing usage in industry and agriculture.1−3 Hg2+ accumulates in the body even at low levels of exposure and leads to long-term irreversible damage and multiple pathologies, including digestive, kidney, brain, and especially neurological dysfunction.4−7 The risk of mercury poisoning has motivated the development of methods to selectively detect, quantify, and adsorb Hg2+ from aqueous solution.8 Fluorescence-based detection methods are especially promising, since they enable the presence of a chemical species to be converted into a light signal change and have the advantage of high sensitivity, convenient operation, and the possibility of easy readout with the naked eye.9−12 While much progress has been made in the fluorescent detection of heavymetal ions,13,14 several challenges still exist for Hg2+-based sensors,15,16 including the need for robust detection in aqueous solutions, minimal cross-sensitivity toward other metal ions, especially Pb2+, short response times, high recyclability, and complete fluorescence quenching upon exposure to low concentrations of Hg2+, which could enable easy visual detection.17,18 Ideally, such sensors would be portable, © XXXX American Chemical Society

inexpensive, convenient to use, and capable of on-site applications. Taken together, the development of luminescent Hg2+ sensors remains an important and challenging task. Porous organic polymers (POPs), which knit organic building blocks into three-dimensional scaffolds, feature excellent porosity, hierarchical pore structure, tunable chemical nature, and good stability.19−25 These advantages render POPs promising sensor materials: the porosity greatly increases the surface area for analyte interaction and amplifies sensitivity;26,27 the hierarchical pore structure enhances mass transfer within the pores and increases detection rates;19,28,29 the tunable structure facilitates the creation of task-specific sites to target analytes;30 and the good stability enables repeated use, including regeneration/recycling of a POP-based sensor that acts via analyte capture.31 Combining the above design elements of POP scaffolds with molecular sensor elements, i.e., a fluorophore and a receptor,32 we aimed to construct a Hg2+ detection platform with a rigid polymer backbone that Received: April 13, 2019 Accepted: June 18, 2019 Published: June 18, 2019 A

DOI: 10.1021/acsami.9b06488 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Synthesis of TPA-POP-CHO Under Mini-Emulsion Conditions and Subsequent Postpolymerization Modification to Afford TPA-POP-TSC (NanoPOP)a

a TPA-POP-TSC was well dispersed in aqueous solution and was highly sensitive to Hg2+. Photos of aqueous TPA-POP-TSC nanoPOP suspensions under UV light (two vials in the middle and right-hand-side sections of the scheme) and the cartoon pictures visualize the significant color change in the fluorescence emission before and after the addition of Hg2+.

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can also serve as a fluorophore and has the capability of appending a receptor with the specific task of Hg2+ capture through strong, yet reversible, binding. A drawback of most reported POPs is that they are often insoluble and cannot be well dispersed in solvents. This limits their applications in liquid systems, especially in aqueous solutions, and can prevent the processing necessary for incorporation into versatile sensing devices, such as films and test papers.33,34 Recently, this drawback has been circumvented by synthesizing nanometer-sized POPs (nanoPOPs) under mini-emulsion conditions,35,36 allowing the solution processability of these materials to be improved significantly without compromising porosity. Accordingly, we aimed to make full use of this method and to explore the power of nanoPOPs in a wider array of detection media, including portable sensing devices. Herein, we describe a solution-processable, porous florescent organic polymer for highly sensitive, fast, and readily visualized Hg2+ detection. We designed a triphenylamine (TPA)-based nanoPOP as the fluorophore. After a postpolymerization, the thiosemicarbazide (TSC)-based moiety was introduced to efficiently and selectively bind Hg2+. We reasoned that this combination would render the nanoPOP task-specific: even at very low analyte concentration, strong interaction of the nanoPOP with Hg2+ would lead to significant fluorescence change. Furthermore, mini-emulsion polymerization in toluene-in-water would produce well-dispersed nanoPOP spheres suitable for aqueous solution-based detection of pollutants and facile processability on paper substrates to afford sensing devices. Therefore, the successful development of this functional nanoPOP introduces an effective strategy that combines several advantages in a sensing performance matrix, including excellent sensitivity, selectivity, fast kinetics, high recyclability, and processability.

EXPERIMENTAL SECTION

Materials and Methods. Unless otherwise stated, reagents were commercially obtained and used without further purification. Dioxane was freshly distilled from Na, and toluene was freshly distilled from CaH2. Characterizations are given in the Supporting Information. Synthesis of TPA-POP-CHO. A solution of TPA-CHO (0.216 g, 0.5 mmol) and tetrakis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)methane (TBPM) (0.206 mg, 0.25 mmol) in degassed toluene (10 mL) was added to a degassed aqueous solution of cetyltrimethylammonium bromide (15 g, 300 mL) under N2, and the mixture was subjected to ultrasonication at 65 °C for 20 min. Aqueous K2CO3 (5 mL, 2 M) was then added and the mixture was ultrasonicated at 65 °C for 20 min and stirred at 80 °C for 72 h. While continuing the stirring at 80 °C and under N2 atmosphere, bromobenzene (100 μL) was added. After 2 h, phenylboronic acid (100 mg) was added and the reaction was allowed to continue for an additional 2 h, with the bromobenzene and phenylboronic acid serving as end-capping reagents. The mixture was then cooled to room temperature and poured into a solution of MeOH/dichloromethane (DCM)/tetrahydrofuran (THF) (50:50:50 mL). The organic layer was collected, the water phase was extracted by DCM three times, the organic layers were combined, and then the volatiles were removed under reduced pressure. The residue was washed with MeOH via Soxhlet extraction and dried under vacuum to afford TPAPOP-CHO as a light yellow powder (134 mg, 62% yield). Synthesis of TPA-POP-TSC. The postpolymerization modification procedure was slightly modified from the synthesis of thiosemicarbazide-based small molecules.37 TPA-POP-CHO (100 mg) was dispersed in EtOH (5 mL), to which 0.2 mL of AcOH and 300 mg of thiosemicarbazide were added. The reaction mixture was refluxed for 6 h and then cooled. The solid was collected by centrifugation, washed with EtOH (3 × 15 mL via centrifugation), and then dried under vacuum to afford a yellow powder. Computational Methodology. The ground-state structures of TPA-POP-TSC segments were computed using the Dmol3 module in Material Studio 6.0. The Perdew−Burke−Ernzerhof method was used to approximate the exchange correlation energy.38 The double B

DOI: 10.1021/acsami.9b06488 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) FTIR spectra for TPA-POP-CHO and TPA-POP-TSC; (b) N2 adsorption−desorption isotherms of TPA-POP-CHO and TPA-POPTSC at 77 K. The solid and open markers represent adsorption and desorption isotherms, respectively. (c) Transmission electron microscopy (TEM) graph of TPA-POP-CHO and (d) TEM graph of TPA-POP-TSC.

the isotherms suggests swelling of the nanoPOP backbone.40,41 The Brunauer−Emmett−Teller (BET) surface area of TPAPOP-CHO was calculated to be 161 m2 g−1 (Figure 1b). Thermogravimetric analysis illustrated the excellent thermal stability of TPA-POP-CHO, which decomposed at 350 °C (Figure S2). The porosity, thermal stability, and presence of aldehyde groups rendered TPA-POP-CHO a good candidate for further postpolymerization modification. The thiosemicarbazide group, known to bind Hg2+,42−44 grafted onto the nanoPOP via postpolymerization modification to afford TPA-POP-TSC. This functionality was included for the specific task of capturing Hg2+ in the nanoPOP pores and in proximity to the fluorophore. The FTIR spectrum of the product material, TPA-POP-TSC, indicates almost complete loss of the carbonyl stretch and the emergence of new bands at 1361 and 1094 cm−1, which correspond to the thiosemicarbazide’s thioamide stretching modes. Furthermore, 13C NMR spectral analysis showed that the aldehyde resonance at 204 ppm was replaced by a peak at 180 ppm (Figure S1). Importantly, the nanosphere morphology was retained after postpolymerization modification (Figure 1d), ensuring the good dispersity and solution processability of the nanoPOP derivative. Not surprisingly, the measured BET surface area decreased relative to the nonfunctionalized material (to 46 m2 g−1, Figure 1b), and the absorption profile at low pressure suggested that micropores were not compromised by postpolymerization modification. Elemental analysis showed that the sulfur and nitrogen contents significantly increased, from negligible level to 5.45% and from 4.26 to 7.50%, respectively, for the TPA-POP-TSC compared to TPA-POPCHO. These data strongly support the assertion that the thiosemicarbazide was successfully installed with high efficiency (Figure 1a). The optical properties of TPA-POP-CHO and TPA-POPTSC were studied by diffuse reflectance UV−vis and photoluminescence spectroscopies. TPA-POP-CHO exhibited a broad absorption band from UV to 420 nm, while the

numerical basis plus polarization was used for valence electrons. The self-consistent framework (SCF) was used to solve the Kohn−Sham equations, and the SCF iteration was assumed to converge when the energy change was lower than 2.57 × 10−3 kJ mol−1. The geometry optimization was assumed to converge when the energy change was lower than 2.7 × 10−4 eV, force was lower than 0.05 eV Å−1, and displacement was lower than 0.005 Å. The binding energy (BE) is calculated as the energy difference in the adsorption process as defined in eq 1 BE = E Hg ‐ POPs − E Hg − E POPs

(1)

where EHg‑POPs is the total energy of the nanoPOPs/Hg sorption system in equilibrium state, and EHg and EPOPs are the total energies of Hg and Hg-free nanoPOPs, respectively. A negative BE value suggests an exothermic adsorption of the Hg into the nanoPOPs.

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RESULTS AND DISCUSSION The triphenylamine (TPA) derivative 4-(bis(4-bromophenyl)amino)benzaldehyde (TPA-CHO) is an efficient light-emitting motif with aggregation-induced emission,39 and the aldehyde group is available to introduce receptors to bind with Hg2+. As illustrated in Scheme 1, TPA-CHO was reacted with tetrakis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)methane (TBPM) by mini-emulsion to afford TPAPOP-CHO. Fourier-transform infrared (FTIR) analysis shows the presence of the aldehyde (1695 cm−1) and phenyl (1590 and 1488 cm−1) functionalities (Figure 1a). 13 C NMR analysis also confirmed the structure (Figure S1). Briefly, resonance at 64 ppm indicates quaternary amine carbon attributable to the tetraphenylmethane unit; the peaks in the range of 147−126 ppm indicate the aromatic backbone, and the 204 ppm resonance indicates the presence of aldehyde. The mini-emulsion conditions gave product with nanosphere morphology and diameters of 100−200 nm (Figure 1c). Amorphous pores were observed with high-resolution TEM imaging (Figure 1c, inset). Nitrogen adsorption experiments at 77 K revealed the type I isotherms for TPA-POP-CHO, suggesting the presence of micropores; the nonclosed nature of C

DOI: 10.1021/acsami.9b06488 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces absorption band for TPA-POP-TSC was 10 nm wider (Figure S3). TPA motifs are known to generate aggregation-induced emission, which is consistent with the blue solid-state emission (peak at 462 nm) observed from TPA-CHO (Figure 2). TPA-

sensing kinetics in water, TPA-POP-TSC microbeads were suspended in a solution of THF/H2O (1:99, v/v) and exposed to 4 μM Hg(ClO4)2, and the time-dependent fluorescence response was measured (Figure S5). The fluorescence intensity quickly decreased to a constant value within approximately 12 s (Figure 3a). It is noteworthy that the TPA-POP-TSC nanoparticles were highly dispersed in the solution, appearing homogeneous fluorescent. The TPA-POP-TSC suspension also led to apparently uniform exposure to Hg2+ across particles, which should increase detection accuracy, reliability, and importantly, rapidity, compared to poorly dispersed or immiscible particles. Indeed, while a number of porous materials with uniform micropores were reported to have analyte response times of minutes or hours,16,46 the hierarchical TPA-POP-TSC pore structure is superior to uniform pore sizes and contributes to rapid analyte diffusion28,47 and fast sensing kinetics. TPA-POP-TSC nanoparticle sensitivity to Hg2+ was evaluated by fluorescence spectroscopy in THF/H2O (1:99, v/v) across a range of Hg(ClO4)2 concentrations (Figure 3b). Almost 90% of fluorescence was quenched at Hg 2+ concentrations of 5 μM. Fluorescence quenching efficiency was quantitatively assessed via the Stern−Volmer (SV) equation (eq 2)

Figure 2. Solid-state fluorescence spectra of TPA-CHO, TPA-POPCHO, and TPA-POP-TSC; inset: photos of TPA-POP-CHO and TPA-POP-TSC powders under UV light.

POP-CHO showed cyan solid-state fluorescence with an emission maximum at 505 nma bathochromic shift relative to monomeric TPA-CHO (Figure 2). Emission from TPAPOP-TSC was slightly blue-shifted relative to TPA-POP-CHO, peaking at 500 nm. With sensor applications in mind, we examined the fluorescence of TPA-POP-TSC in aqueous solution, which revealed that TPA-POP-TSC dispersed in THF/H2O (1:99, v/v) exhibited cyan fluorescence centered at 485 nm (Figure S4) and suggested that this material would be suitable for Hg2+ sensing in aqueous systems. The feasibility of detecting Hg2+ in water is of distinct practical significance and compares well to materials developed to detected Hg2+ in organic solvents.17,45 To study the Hg2+

(I0/I ) = 1 + K sv[Q]

(2)

where [Q] is the molar Hg2+ concentration, I0 and I are the fluorescence intensities before and after addition of Hg2+, respectively, and Ksv is the Stern−Volmer constant (M−1). The Stern−Volmer plots exhibited linearity at low Hg2+ concentration, and the Ksv value was calculated to be 1.01 × 106 M−1 (Figure S6). Remarkably, this value is higher than most porous material-based Hg2+ sensors, including NOP-28@2 (3.7 × 104 M−1),18 HCMP-1 (1.1 × 104 M−1),17 and PCN-224 (6.4× 105

Figure 3. (a) Fluorescence intensity of TPA-POP-TSC nanoPOP in THF/H2O (1:99, v/v) solution (0.1 mg mL−1) as a function of exposure time to 4 μM of Hg2+. (b) Fluorescence spectra of TPA-POP-TSC nanoPOP in THF/H2O (1:99, v/v) solution upon addition of various concentrations of Hg2+; inset: photos of TPA-POP-TSC nanoPOP in THF/H2O (1:99, v/v) solution under ambient light and UV light, before and after addition of 10 μM Hg2+, respectively. (c) Fluorescence intensity of TPA-POP-TSC nanoPOP in THF/H2O (1:99, v/v) solution following exposure to various metal cations, with intensity relative to the nonexposed sample. (d) Cycling tests of TPA-POP-TSC nanoPOP in Hg2+ detection. D

DOI: 10.1021/acsami.9b06488 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces M−1)48 (Table S2). The detection limit, 3.7 ppb, outperforms all previously reported covalent organic frameworks (COFs) and other POP sensor materials, including COF-LZU8 (25.0 ppb)45 and NOP28@2 (12.0 ppb).18 COF-LZU8 and NOP28@2 were also difficult to disperse well in water and instead were reported to detect Hg2+ in organic solvent. Although RuMOFs have been reported to have very low detection limits (near 0.5 pM), this material is not robust, as they decomposed while sensing, required long response times (up to 12 h), and could not be recycled.49 When exposed to 10 μM Hg2+, the color of TPA-POP-TSC solution exhibited negligible change under ambient light, while the fluorescence was almost completely quenched and the difference is clearly visible by the naked eye (Figure 3b, inset). This simple and easy way to visually identify Hg2+ is attractive for any in-field applications, where sophisticated fluorescence instruments may not be available. In addition to sensitivity, the nanoPOP material also exhibits excellent selectivity and recyclability as a Hg2+ sensor. TPAPOP-TSC showed negligible fluorescence response to other common cations (including Na+, K+, Ca2+, Ba2+, Fe3+, Cr3+, Co2+, Pb2+, Cd2+, Ag+, Cu2+) (Figures 3c and S7). We expect the size complementarity and soft acid/base pairing of Hg2+ with thiosemicarbazide strongly drive selective binding. The mercury ion is comparatively much larger (Hg2+ radius = 102 pm, Cu2+ 73 pm, Fe3+ 55 pm, Cd2+ 95 pm, Cr3+ 62 pm, Co2+ 75 pm),50 and its equilibrium constant of the thiourea−Hg2+ (lgβ = 22.1) complex is significantly higher than other metal cations, including comparatively soft Ag+ (lgβ = 9.7), Cd2+ (lgβ = 4.3), Cu2+ (lgβ = 10.4), and Pb2+ (lgβ = 0.5), indicative of its particularly strong bonding with thiosemicarbazide.51−55 We further measured the Hg2+ sensing performance of TPA-POPTSC under competitive environment and found that the sensitivity toward Hg2+ was barely influenced by other common metal cations that coexist in the solution, such as Na+, K+, Ca2+, Ba2+, Fe3+, Cr3+, Co2+, Pb2+, Cd2+, Ag+, and Cu2+ (Figure S8). We also tested the stability of the TPA-POP-TSC nanoPOP at different pH values. The results show that no significant changes are found in its quenching efficiency in the pH range of 1−10. When pH > 10, the quenching efficiency gradually decreases due to the sediment of Hg(OH)2 (Figure S9). We carried out contrast tests using TPA-POP-TSC in pure water and in real river water. The Hg2+ quenching efficiencies are almost identical (Figure S10). The main cations of Na+, K+, Ca2+, Mg2+, Al3+, Fe3+ in river water (Table S3) showed little effect on the fluorescence performance of TPAPOP-TSC. The quenching efficiency of TPA-POP-TSC for Hg2+ in river water is 90%, only slightly lower than 91% in pure water (Figure S10). Unlike soluble and/or small-molecule fluorescent detection systems, which can be difficult to recover after the sensing event, cross-linked nanoPOPs may be easily separated from the analyte. The negligible color change under ambient light upon addition of Hg2+ (Figure 3b, inset) also suggested that the TPA-POP-TSC material, even when bound to Hg2+, was stable and that regeneration was possible. Remarkably, by simply washing with Na2S (0.01 M), TPAPOP-TSC was fully restored to its original fluorescence. Subsequent Hg2+ detection/regeneration indicated that the sensing ability of TPA-POP-TSC barely deteriorated even after several cycles (Figures 3d and S11), demonstrating that this detection platform has excellent reusability. The excellent sensing performance of TPA-POP-TSC nanoPOP inspired us to further explore its potency as a

sensing device. Considering the high solution processability of TPA-POP-TSC nanoPOP, a very simple, low-cost, and easyto-use sensor device can be made by uniformly depositing the sample on a suitable substrate, such as paper, to form fluorescent test strips. For this purpose, a TPA-POP-TSC nanoPOP suspension in THF (0.05 mg mL−1) was dropped onto a filter paper which was subsequently dried under vacuum to form a visually homogenous coating. Spot tests using this test paper as the detection device were carried out. Droplets of Hg2+ solutions of different concentrations were placed on the detection strips, the strips were allowed to dry, and the remaining emission-quenched areas were observed by casual inspection. Exposure to a blank solution had no readily visible effect on the detection strip (Figure S12). However, following exposure to a Hg2+ concentration of 1.0 × 10−6 M, the quenching area (letter “H” in Figure 4 and spot in Figure S13)

Figure 4. Photos of TPA-POP-TSC nanoPOP deposited paper strips under UV light upon addition of Hg2+ at different concentrations: 10−6, 10−5, and 10−4 M Hg2+ solutions were used to write H, 2, and O, respectively.

became observable by the naked eyefor Hg2+ concentrations of 1.0 × 10−5 M and higher, the quenching areas (“2” and “O” in Figure 4 and spots in Figure S13) became highly visible. The visible limit of detection for the paper strips could be quantified56 to be 4 ng cm−2. The test strips are also extremely simple and cost-effective to produce, as each requires less than 25 μg of nanoPOP material, enabling the production of approximately 4000 test strips from a 100 mg nanoPOP sample. Hence, this simple, low-cost, and easy-to-use detection system enabled the direct visualization of aqueous Hg2+ without instrumentation. Needless to say, the paper strips also have the advantage of convenient handling. To understand the extent of interactions between the sensor and Hg2+, and the detection mechanism of TPA-POP-TSC, we performed X-ray photoelectron spectroscopy (XPS) experiments on Hg2+-loaded nanoPOP samples. As shown in Figures S15 and S16, four N species were evident for TPA-POP-TSC. One N 1s peak with binding energy of 400.8 eV was ascribed to the Schiff base (the NC bond). This was shifted to 401.2 eV upon exposure to Hg2+ (100 ppm), which is consistent with N 1s donation to Hg2+ upon complexation. Similarly, S 2p2/3 peaks also underwent a blue shift from 161.9 to 162.5 eV, consistent with the interaction between Hg2+ and S upon complexation. These shifts are typical for Hg2+ binding,57 and consistent with the data observed in the FTIR spectra, where the N−CS bands at 1360 and 1088 cm−1 shifted to 1400 and 1115 cm−1, respectively, and decreased in intensity (Figure 5a). Having located the adsorbed Hg2+ on the thiosemicarbazide, we then performed DFT calculations (see details in Computational Methodology). The BE of Hg2+ and one TPAPOP-TSC segment was found to be −29.84 kJ mol−1 (Figure 5b), which is remarkably strong.58−61 Thus, the data suggest that the S and N atoms coordinate to Hg2+, increase the E

DOI: 10.1021/acsami.9b06488 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. (a) FTIR spectra of TPA-POP-TSC before and after soaking in 100 ppm Hg2+ solution; (b) binding energy of Hg2+ with one thiosemicarbazide group.

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CONCLUSIONS In this work, we describe the design, synthesis, and characterization of a new platform for very simple, fast, convenient, low-cost, and highly sensitive Hg2+ detection by the naked eye. TPA-POP-TSC nanoPOP enables Hg2+ detection in aqueous systems. The design of TPA-POP-TSC nanoPOP includes a triphenylamine-based backbone that serves as the fluorophore and a thiosemicarbazide receptor to efficiently bind Hg2+. We suggest that the design features of TPA-POP-TSC nanoPOP synergize to give the observed excellent sensing profile: (1) The thiosemicarbazide sites specifically interact with Hg2+, contributing to the high specificity over common metal ions (Na+, K+, Ca2+, Ba2+, Fe3+, Cr3+, Co2+, Pb2+, Cd2+, Ag+, Cu2+). (2) The porosity, swelling, and abundant active sites give rise to strong fluorescence quenching with excellent sensitivity (Ksv = 1.01 × 106 M−1; detection limit: 3.7 ppb; BE = −29.84 kJ mol−1). (3) The hierarchical pore structure and nanometer-sized POP particles facilitate rapid mass transfer, making real-time detection possible (a few seconds). (4) The good stability of the material and the regeneration of active sites make this sensor reusable and cost-effective. (5) The TPA-POP-TSC nanoPOP was synthesized under mini-emulsion system and obtained as solution-processable nanoparticles. This processability allowed the development of inexpensive and easyhandling Hg2+ test strips that produce a response to as low as 4 ng cm−2 that is visible to the naked eye. Taken together, the new, sensitive, selective, convenient, inexpensive, and portable detection platform represents a significant advance in Hg2+ sensing.

effective positive charge on these atoms, increase the vibrational frequency, and result in an overall tightly bound complex. The DFT calculations also assisted us in identifying the likely quenching mechanism that gives rise to the sensing performance of the nanoPOP. Emission in TPA-POP-TSC stems from the TPA motif (Figure S17). We calculated the frontier orbital energy level for both fluorophore and receptor, and found that before exposure to Hg2+, the lowest occupied molecule orbital (LUMO) energy of the receptor (−2.034 eV) was higher than that of the fluorophore (−2.089 eV), so electron transfer of the exited electrons from fluorophore to receptor is unlikely to occur (Figure 6a). Upon complexation

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b06488. Figure 6. Frontier orbital energy diagrams and electron-transfer paths for segments of TPA-POP-TSC before (a) and after (b) addition of Hg2+.

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with Hg2+, the LUMO energy of the receptor decreased drastically (to −3.084 eV) and became significantly lower than that of fluorophore (Figure 6b). This enables photoexcited electron transfer from the fluorophore LUMO to the receptor LUMO, resulting in strong fluorescence quenching.

Materials and characterizations: 1H NMR spectra; solidstate 13C NMR spectra; thermogravimetric analysis images; nitrogen adsorption isotherms; UV−vis diffuse reflectance spectroscopy; photoluminescence measurements; FTIR analysis; elemental analysis; and XPS data of the materials under study (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.H.). *E-mail: [email protected] (J.L.). F

DOI: 10.1021/acsami.9b06488 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces ORCID

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Jun Hu: 0000-0002-3020-0148 Jing Li: 0000-0001-7792-4322 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the Department of Energy, Basic Energy Sciences, and Division of Materials Sciences and Engineering through Grant No. DEFG02-08ER-46491, the National Natural Science Foundation of China (Nos 21878076, 21676080). Y.L. acknowledges the China Scholarship Council (CSC) for supporting his study at Rutgers University.

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DOI: 10.1021/acsami.9b06488 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX