Diketopyrrolopyrrole Amphiphile-based Micelle ... - ACS Publications

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Diketopyrrolopyrrole Amphiphile-Based Micelle-Like Fluorescent Nanoparticles for Selective and Sensitive Detection of Mercury(II) Ions in Water Kaixuan Nie,† Bo Dong,† Huanhuan Shi,† Zhengchun Liu,*,† and Bo Liang*,‡ †

Hunan Key Laboratory for Super Microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha, Hunan 410083, People’s Republic of China ‡ State Engineering Laboratory of Highway Maintenance Technology, Changsha University of Science and Technology, Changsha, 410114, People’s Republic of China S Supporting Information *

ABSTRACT: A technique for encapsulating fluorescent organic probes in a micelle system offers an important alternative method to manufacture watersoluble organic nanoparticles (ONPs) for use in sensing Hg2+. This article reports on a study of a surfactant-free micelle-like ONPs based on a 3,6-di(2thienyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (TDPP) amphiphile, (2(2-(2-methoxyethoxy)ethyl)-3,6-di(2-thiophyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (NDPP) fabricated to monitor Hg2+ in water. NDPP was synthesized through a simple one-step modification of a commercially available dye TDPP with a flexible and hydrophilic alkoxy. This study reports, for the first time, that TDPP dyes can respond reversibly, sensitively, and selectively to Hg2+ through TDPP−Hg−TDPP complexation, similar to the well-known thymine(T)−Hg−thymine(T) model and the accompanying molecular aggregation. Interestingly, transmission electron microscopy (TEM) and dynamic light scattering (DLS) confirmed that, in water, NDPP forms loose micelle-like fluorescent ONPs with a hydrohobic TDPP portion encapsulated inside. These micelle-like nanoparticles offer an ideal location for TDPP−Hg complexation with a modest molecular aggregation, thereby providing both clear visual and spectroscopic signals for Hg2+ sensing. An estimated detection limit of 11 nM for Hg2+ sensing with this NDPP nanoparticle was obtained. In addition, NDPP ONPs show good water solubility and high selectivity to Hg2+ in neutral or alkalescent water. It was superior to most micellebased nanosensors, which require a complicated process in the selection or synthesis of suitable surfactants. The determinations in real samples (river water) were made and satisfactory results were achieved. This study provides a low-cost strategy for fabricating small molecule-based fluorescent nanomaterials for use in sensing Hg2+. Moreover, the NDPP nanoparticles show potential ability in Hg2+ ion adsorption and recognization of cysteine using NDPP-Hg composite particle.

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nanoparticles, careful selection or synthesis of suitable surfactant (or surfactant-like macromolecule) and concentration control are essential.9,14−16 Thus, the fabrication of surfactant-based ONPs is complicated and inefficient. Therefore, surfactant-free micellar ONPs are an alternative approach to meet the increasing demand for low-cost and facile ONPbased nanoprobes. Surfactant-like amphiphilic molecules that possess both hydrophobic and hydrophilic units have shown a strong tendency to form self-assembled nanostructures in aqueous solutions; these molecules are thus a possible way to produce surfactant-free micelle ONP-based nanoprobes.17 To date, the study of small molecules is challenging, although many micellar ONPs based on amphiphilic macromolecules17−20 have been made. Recently, Zhu et al. produced a novel micelle-like

rganic nanoparticles (ONPs) have received widespread attention in the fields of sensing,1 imaging,2 and drug delivery3 because of their unique physical and chemical properties.4,5 Recently, numerous micellar ONP-based nanoprobes have been developed and have shown excellent water solubility, biocompatibility, and stability.6−8 These ONPs generally contain two parts, namely, a hydrophobic core and a hydrophilic shell. As a special nanoparticle, these micellar ONPs make the hydrophobic molecule-based probes stable, enriched, and dispersed inside its characteristic inner nanospace, which benefit the utilization of some useful but rigid hydrophobic dyes (e.g., pyrene) in sensing.9,10 To prepare micellar ONPs, commercial surfactants7,11 and surfactant-like amphiphilic block copolymers12,13 are widely used to fabricate the hydrophilic shell. However, this organic probe/surfactant heteroaggregate-based strategy used to fabricate micellar ONPbased nanoprobes also causes several problems. Given that the variety and concentration of surfactants have important effects on encapsulated probes (or analytes) and the particle size of © XXXX American Chemical Society

Received: October 31, 2016 Accepted: February 6, 2017 Published: February 6, 2017 A

DOI: 10.1021/acs.analchem.6b04258 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry

Scheme 1. Proposed Mechanism for the Detection of Hg2+ Using NDPP-Based ONPs

nanoparticle using quinoline−malononitrile derivatives (a small molecule) to recognize bovine serum albumin (BSA) and trypsin.21 These nanoparticles have unique loose micelle-like nanostructures with large internal free volume. They can effectively suppress the aggregation of probe molecules and assist the entrance of analytes into the nanoparticles. Their findings demonstrate the feasibility of fabricating surfactant-free micellar ONP-based nanoprobes using the analyte-triggered aggregation and disaggregation of encapsulated hydrophobic small molecules. Nanoprobes created with this approach are scarce. Even at low concentrations, mercury is highly toxic to human health and the environment; thus, it has gained extensive attention over the years.22,23 Probes based on organic dyes,24,25 DNA,26,27 and gold nanoparticles28,29 have shown good sensitivity and selectivity in Hg2+ sensing. However, these tools also have several potential pitfalls, such as frightening synthetic efforts and light bleaching for organic dye probes and high costs for DNA- and gold nanoparticle-based systems. More low-cost probes suitable for large-scale use are required to monitor the increasing severity of mercury pollution. Facile micelle-based ONPs offer an alternative solution to the problem. Considering the potential interference from surfactants, we aim to produce surfactant-free micellar OPNs based on small molecule. To this end, we selected a powerful Hg2+philic organic molecule similar to thymine (T), which is one of the most effective receptors for Hg2+,30,31 as the receptor for Hg2+. In addition, the selected molecule must be simple, stable, affordable, and have excellent optical properties. Herein, a smart diketopyrrolopyrrole (DPP) dye with high fluorescence was studied. DPP dyes possessing a hydrophobic skeleton have multiple sites for hydrophilic modification.32,33 They are currently one of the most widely used commercial building blocks in organic photovoltaics34 and organic field-effect transistors35 because of their excellent coplanarity, stability, and optical properties. Most importantly, DPP dye possessing a lactam moiety can bind to several heavy metal ions via analogous T−Hg−T complexation, which makes them potential receptors for Hg2+.36,37 In the current work, two probes, monomeric 3,6-di(2thienyl)-2,5-dihydropyrrolo-[3,4-c]-pyrrole-1,4-dione (TDPP) and (2-(2-(2-methoxyethoxy)ethyl)-3,6-di(2-thiophyl)-2,5dihydropyrrolo[3,4-c]pyrrole-1,4-dione (NDPP)-based micelle OPNs, were investigated for sensing Hg2+. To the best of our knowledge, this is the first report about the detection of Hg2+ using TDPP−Hg−TDPP complexation. The complexation increases the rigidity of the molecular structure and causes molecular aggregation. Finally, efficient aggregation between monomeric TDPP leads to dramatic fluorescence quenching and an observable color change from yellow to blue. However, raw TDPP requires substantial amounts of methanol in the sensing system and suffers from Ag+ interference. In addition, TDPP undergoes uncontrollable aggregation in Hg2+ solutions, which induces a nonlinear change of the optical signal. Compared with TDPP, the amphiphilic NDPP molecule comprising a hydrophobic TDPP unit, and a hydrophilic alkoxy can form self-assembled micelle-like ONPs. We consider that the TDPP−Hg complexation and molecular aggregation was well-controlled inside the micelle-like nanostructure (Scheme 1). Such characteristics contribute to the highly selective response to Hg2+, with a wide linear response range of 0−4 μM and a low detection limit of 11 nM. This result is better than most micelle-like nanoprobes (Table S1). This

work demonstrated the potential of using TDPP-based loose micellar ONPs in Hg2+ sensing and offered a reliable design of small molecule-based, water-soluble, fluorescent ONPs.

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EXPERIMENTAL SECTION Instrumentation. Results of the 1H NMR spectra were recorded on a Bruker DRX-500 spectrometer using DMSO-d6 as the solvent and tetramethylsilane as the internal standard. A Lambda 35 UV/vis spectrometer (PerkinElmer) was used for absorption spectroscopy measurements, and the fluorescence data were recorded on a HITACHI F-4500 fluorescence spectrophotometer. The pH was measured with a Model pHs3C meter (Shanghai, China) at room temperature. Results of the Fourier transform infrared (FTIR) spectra were collected on a PerkinElmer Spectrum 2000 FTIR spectrophotometer with a range of 4000−4500 cm−1. X-ray photoelectron spectroscopy (XPS) data were obtained through an ESCALAB 250Xi X-ray photoelectron spectrometer using 300 W Al−K alpha radiations. Scanning electron microscopy (SEM) images were captured on FEI Quanta 200 field emission scanning electron microscope (operating at 20 kV) equipped with an Oxford Inca Energy Dispersive X-ray system for the elemental mapping of mercury. TEM (HRTEM, JEOL JEM-1400) was used to observe the morphology and microstructures of the nanoparticles of NDPP. A Malvern Nanosizer S instrument was used to measure the hydrodynamic size of nanoparticles at room temperature. Hg2+ in real samples was detected on an Agilent 7700 inductively coupled plasma mass spectrometer (ICP-MS). Chemicals. All chemicals and reagents, unless stated otherwise, were used as received from commercial sources without further purification. The TDPP and cysteine (Cys) were purchased from Sigma-Aldrich. The CH3OH for spectral detection was an HPLC reagent with no fluorescent impurities. The cations (Hg2+, Ag+, Pb2+, Cu2+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, and Cd2+) were prepared from their perchlorate salts, whereas Al3+ was made from AlCl3. The anions (Cl−, Br−, I−, CH3COO−, HCO3−, NO3−, H2PO4−, HPO42−, and SO42−) were prepared from their sodium salts, whereas F− was prepared from both tetrabutylammonium and sodium salt. Double distilled water was used throughout the experiment. Synthesis of NDPP. The compound 3,6-di(2-thienyl)-2,5dihydropyrrolo[3,4-c]pyrrole-1,4-dione (300 mg, 1.0 mmol) in B


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flexible alkoxy group gave the NDPP−Hg complex with good solubility in common solvents. Different Self-Aggregation Behaviors of TDPP and NDPP. Small fluorescent molecules (e.g., squaraine or perylene) with rigid plane and π-conjugated structures commonly have a strong tendency for aggregation in aqueous solutions, displaying aggregation-evoked optical changes.38 TDPP with similar rigid structures can undergo strong intermolecular π−π stacking and aggregation.39,40 Thus, the stimuli-triggered aggregation of TDPP may be used for sensing. Considering the fact that TDPP is extremely hydrophobic, to eliminate possible interference, precautions must be taken against the possible self-aggregation of TDPP dyes before sensing. As shown in Figure 1a, TDPP shows a strong characteristic absorption band at 523 nm and a shoulder band at 486 nm. By

N,N-dimethylformamide (DMF; 10 mL) was treated with potassium tert-butoxide (285 mg, 2.5 mmol) for 0.5 h at room temperature. Then, 1-bromo-2-(2-methoxyethoxy)ethane (300 mg, 1.6 mmol) was slowly added, and the mixture was stirred for 6 h and poured into water (150 mL) and extracted thrice with dichloromethane (20 mL). All the solvent in the organic layer was removed under reduced pressure. The crude product was purified through silica gel column chromatography using dichloromethane/ethyl acetate (2:1, v/v) as the eluent, and NDPP was obtained as a red solid (242 mg, 60.5%). 1H NMR (500 MHz, DMSO-d6, δ ppm) results are as follows: 3.18 (s, 3H), 3.37−3.36 (m, 2H), 3.52−3.51 (m, 2H), 3.65−3.63 (t, 2H), 4.16−4.14 (t, 2H), 7.38−7.32 (m, 2H), 8.04−8.02 (t, 1H), 8.26−8.25 (d, J = 3 Hz, 1H), 8.65−8.64 (d, J = 3 Hz, 1H), and 11.40 (s, 1H). Computational Methods. The geometric optimization of the ground state of TDPP and the TDPP−Hg complex was obtained at the B3LYP/6-31G(d) level using Gaussian 09 programs suits. In the calculation, the 6-31G basis set was used for the H, C, N, O, and S atoms, and the LANL2DZ basis set was employed for the Hg atom. Procedures for Sensing. The TDPP (10 μM) and NDPP (10 μM) in CH3OH/H2O (1:1, v/v) solution (containing 1% DMF) and water (containing 0.1% DMF) were added into 5 mL plastic sample tubes. The Hg2+ (10 mM) and other metal ions (100 mM), including Ag+, Pb2+, Cu2+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cd2+, and Al3+, were added and mixed thoroughly for 60 min at room temperature. Then, all the UV−vis and fluorescence spectra were recorded at 25 °C unless another temperature was noted. The color changes were documented under natural light as well as under a UV lamp (365 nm). Before sensing, TDPP (1 × 10−3 M) and NDPP (1 × 10−2 M) were prepared in DMF and stored in the refrigerator. Throughout the measurement, the introduction of DMF in the sensing system was negligible. Preparation of TDPP−Hg (NDPP−Hg) Complex. A total of 30 mL of 10 μM TDPP was first prepared in CH3OH/H2O (1:1, v/v), followed by the addition of 3 mL of 100 μM Hg2+ aqueous solution with powerful stirring at 50 °C. The mixed solution was stirred for 60 min before being left to stand overnight. The precipitate was then filtered out and dried in a vacuum at 60 °C to yield a DPP−Hg complex as a bluish violet solid. The solid was used for further characterization. The NDPP−Hg complex was prepared identically to the method described above.

Figure 1. (a) UV−vis absorption spectra of TDPP (10 μM) at 25 °C with different ratios of CH3OH/H2O; (b) DLS data of NDPP in methanol and water; (c) TEM images of NDPP nanoparticles in water. Inset: Tyndall effect of NDPP in water (up) and the aggregation of nanoparticles (bottom); (d) UV−vis absorption spectra of NDPP (10 μM) at 25 °C with different ratios of CH3OH/H2O.

increasing the water ratio in the TDPP solution, the absorption intensities at 486 and 523 nm decreased, and a new absorption band at a longer wavelength (558 nm) emerged. Similar changes were observed when the temperature of the solution was decreased (Figure S1). These changes imply the strong tendency of TDPP molecules for aggregation. 41 The aggregation-induced microparticles of TDPP were confirmed through simple filtration with 0.22 μm membrane (Figure S2). After filtration, red residue was observed in the filter head; simultaneously, no absorption band at 558 nm was recorded in the filter liquor. These results show that rigid plane TDPP can produce an aggregation-related new absorption band in an aqueous solution. The intrinsic aggregation of TDPP before sensing might interfere with Hg2+ detection. Thus, a mixed solvent of methanol and water with a ratio of 1 to 1 (v/v) was used to maintain monomeric TDPP. On the other hand, the presence of self-aggregated nanoparticles of amphiphilic NDPP in water was confirmed through DLS. Results indicated that the diameter of nanoparticles was about 179 nm, with a polydispersity index of 0.35 (Figure 1b). In addition, TEM and Tyndall effect also revealed

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RESULTS AND DISCUSSION Design and Synthesis. The DPP-based probes designed here use both potential TDPP−Hg complexation and molecular aggregation. The amphiphilic NDPP was prepared via attaching one hydrophilic alkoxy to a nitrogen atom of hydrophobic TDPP. NDPP is supposed to form a selfassembled loose nanostructure with an alkoxy unit exposed to water and TDPP units dispersed inside the core. DLS and TEM were used to study formation of these surfactant-free nanoparticles. The chemical structure of the NDPP molecule was characterized through 1H NMR spectroscopy. The TDPP− Hg complex and NDPP−Hg complex were prepared directly by mixing TDPP/NDPP and Hg2+ in methanol/water solution. The TDPP−Hg complex is insoluble in common organic solvents and deuterium oxide, thereby increasing the difficulty of relevant characterization. Fortunately, the introduction of a C


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Figure 2. (a) 1H NMR spectra of NDPP and NDPP−Hg complex in DMSO-d6. (b) XPS spectra of NDPP (bottom) and NDPP−Hg complex.

Figure 3. (a) Absorption spectra of NDPP−Hg complex (2 μM) in different solvents, including dichloromethane/methanol at 1:1 ratio (black), methanol (red), methanol/water = 1:1 (green), and water (blue); (b) optimized geometries of TDPP and relative complexes. Hg, C, N, O, S, and H atoms are represented as white-gray, gray, blue, yellow, and white, respectively; (c, d) DLS data of TDPP and NDPP in the presence of increasing concentration of Hg2+.

aggregation-caused quenching effect (ACQ). As shown in Figure S3, TDPP nearly lost its fluorescence in water. However, NDPP still maintain high fluorescence intensity (only lost 35%) in water with a fluorescence quantum efficiency (ΦF) of 0.28. (rhodamine B in water as the standard with ΦF of 0.31, the absorbance of the solutions was kept around 0.05 to avoid internal filter effect). It is better than many reported fluorescent ONPs,1 which implied that the micelle-like NDPP nanoparticles can effectively restrain the ACQ effect. Possible Sensing Mechanism. Based on the strong binding affinity of unprotonated lactam-N in DPP dyes for transition metal ions,36,37 two DPP dyes can possibly form a DPP−Hg complex. As expected, the NDPP−Hg complex was synthesized easily and determined via 1H NMR spectroscopy (TDPP−Hg complex has poor solubility even in dimethyl sulfoxide, so its 1H NMR data was not recorded). As shown in Figure 2a, a distinct signal (singlet) at δ 11.39 ppm corresponds to the −NH proton of NDPP. Nevertheless, the signal for the −NH proton in the NDPP−Hg complex disappeared, demonstrating the deprotonation of the lactam moiety and the formation of a N−Hg−N bond, which is similar to that involved in the well-known thymine(T)−Hg−T model. The NDPP−Hg complexes were also confirmed through XPS

the formation of these nanoparticles (Figure 1c). As shown in Figure 1c, the aggregation of NDPP nanoparticles was detected, and the nanoparticles have a smaller diameter (ca. 30 nm) than those recorded through DLS. The different sample preparation processes of TEM and DLS may explain the changes in particle size.21 Generally, the DLS sample is conducted directly in the mother solution so that the NDPP possessing the original nanostructure shows a large particle size. However, preparation of TEM sample required solution evaporation, which might have led to the contraction of loose nanostructures and the destruction of this nanoparticle. We consider that the significant change of particle size recorded through DLS and TEM provides direct evidence of the loose nanostructure of NDPP nanoparticles in water. Compared with TDPP, although decline in the characteristic absorption intensity was observed, no aggregation-related new absorption band was induced by these NDPP nanoparticles (Figure 1d). These results indicate again that TDPP cores are well-dispersed inside the loose nanoparticle. These NDPP-based nanoprobes with loose structure will benefit the entrance of Hg2+ and the latter TDPP−Hg complexation. It is inevitable that both TDPP and NDPP undergo fluorescent quenching in aqueous solution due to the D


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Figure 4. UV−vis absorption spectra (dotted lines) and fluorescence spectra (solid lines) of (a) TDPP and (b) NDPP with and without the addition of Hg2+. Inset: color changes upon addition of Hg2+ under natural light and UV lamp (365 nm). Red lines stand for the absorption/fluorescence spectra of DPP solution after addition of Hg2+; λex are 486 and 496 nm for TDPP and NDPP..

Figure 5. Color changes in (a) TDPP (10 μM) in CH3OH/H2O (1:1, v/v) and (b) NDPP (10 μM) in water with increasing Hg2+ concentrations. The concentrations of Hg2+ from left to right are 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 μM. UV−vis absorption spectra of (c) TDPP (10 μM) in CH3OH/ H2O (1:1, v/v) and (d) NDPP (10 μM) in water upon addition of Hg2+ from 0 to 1.0 equiv.

tight aggregation in aqueous solutions. This conclusion implies that NDPP−Hg complexation and the accompanying molecular aggregation work together effectively in sensing Hg2+. Calculations of density functional theory (DFT) of TDPP and TDPP−Hg complex were performed to clarify the influence of DPP−Hg complexation on molecular structure. Figure 3b shows that the optimized minimum-energy configuration of TDPP molecules is nearly planar. After the introduction of Hg atoms, a slight backbone twist (a dihedral angle of 1°) was observed between the two TDPP units in the TDPP−Hg complex. DFT calculations indicated that, unlike individual DPP molecules, the DPP−Hg complex possesses a more rigid skeleton. The enhancement of molecular rigidity may greatly promote molecular aggregation and stacking, bringing about the changes of optical signals for sensing. This assumption was confirmed by previous works using rigid perylenebisimide in Hg2+ sensing.42,43 Furthermore, the Hg2+triggered aggregation of monomeric TDPP molecules was thoroughly confirmed through DLS measurements (Figure 3c,d), which were consistent with the DFT calculations. Importantly, the filtration experiments demonstrated that the formation of nanoparticles is the direct cause of the appearance of the new absorption band (Figure S6). In addition, we found

measurement (Figure 2b). Mercuric perchlorate was used as the mercury source in this study. Distinct characteristic peaks corresponding to C 1s, O 1s, N 1s, and Hg 4f were observed in both complexes and are shown in Figure 2b; however, no signal was attributed to chlorine, indicating the formation of NDPP− Hg complex. The difference between TDPP and TDPP−Hg complex is similar to that described above (Figure S4). Additionally, FTIR data of DPP−Hg complex were investigated (Figure S5). Compared with DPP dyes, the IR spectra of both complexes showed similar changes in lactam because of their direct involvement in complex. The strong absorption band of vCO in the lactam of TDPP and NDPP shifted to a lower wavenumber by 48 and 32 cm−1. However, the formation of NDPP−Hg complex may not be the only stimulant for changing the optical properties of NDPP, and the effect of the solvent is a non-negligible factor. As shown in Figure 3a, the NDPP−Hg complex (2 μM) showed a clear double peak, a result that is similar to that of NDPP in benign solvents, such as dichloromethane, methanol, and even water. However, an obvious aggregation-related absorption band (similar to that of TDPP in water) of the NDPP−Hg complex was observed only in water. These results show that the monomeric NDPP−Hg complex has an enhanced tendency for E


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Figure 6. Fluorescence spectra of (a) TDPP (10 μM, λex = 486 nm) in CH3OH/H2O (1:1, v/v) and (b) NDPP (10 μM, λex = 496 nm) in water upon the addition of different amounts of Hg2+ (0−1.0 equiv). Inset: Plot of fluorescence intensity at λmax as a function of the concentrations of Hg2+ (λ max are 537 and 546 nm for TDPP and NDPP).

Figure 7. Fluorescence responses of (a) TDPP (10 μM) and (b) NDPP (10 μM) to Hg2+ (10 μM), Ag+ (100 μM), and other metal ions (1 mM). F0 is the fluorescence intensity of the TDPP/NDPP solution, and ΔF is the change in fluorescence intensity upon addition of the metal ions.

maximum absorbance intensity decreased, and a new absorbance located at 580 and 615 nm for TDPP and NDPP, respectively, emerged and increased. This increase demonstrates that increasing numbers of TDPP/NDPP molecules are aggregating. An isosbestic point was clearly observed at approximately 545 nm for both probes, which implies the conversion of free molecules into aggregates. In addition, molecular aggregation also resulted in fluorescence quenching of TDPP and NDPP (Figures 6 and S8). The fluorescence intensity of TDPP at 537 nm sharply decreased when the Hg2+ was changed from approximately 0.1 equiv to 0.3 equiv. Then, a mild decrease was observed when Hg2+ was greater than 0.3 equiv. Finally, the fluorescence was totally quenched. A linear relationship was observed between the fluorescence intensity and concentration of Hg2+ lower than 1 μM (Figure S9). The limit of detection (LOD), defined by the equation LOD = 3σ/s, where σ is the standard deviation of blank signals and s is the slope of the calibration curve, was calculated to be 42 nM. This value satisfies the general standard of wastewater discharge and pretreatment regulations of mercury (50 nM).44 In contrast to TDPP, the aggregation behavior of NDPP caused by increasing Hg2+ concentration was gradual and had no intense decrease in fluorescence intensity at 546 nm (Figure 6b). The detection limit of Hg2+ through NDPP was calculated to be 11 nM; this result is better than that of previous surfactant-essential micelle-based ONPs.9,10,45,46 Considering its low-cost and simple preparation

that the introduction of Hg into the NDPP molecular structure induced obvious fluorescence quenching only in water (Figure S7). But in other organic solvent, NDPP−Hg complex still showed strong fluorescence as compared to NDPP. It indicates that the NDPP−Hg complex has a strong tendency for aggregation in water and undergoes strong aggregation-induced fluorescent quenching. Apparently, both DPP−Hg complexation and the accompanying molecular aggregation allowed the optical detection of Hg2+ using DPP dyes. Hg2+ Sensing. The responses of two probes to Hg2+ were recorded via UV/vis and fluorescence spectrophotometer. As shown in Figure 4, Hg2+ can produce an aggregation-evoked new absorption band and quench the fluorescence as well. Thus, both monomeric TDPP molecules and NDPP nanoparticles can recognize Hg2+ through colorimetric and fluorescent determination. The feasibility of two probes for mercury sensing was further evaluated by studying their sensitivity, selectivity, and adaptability. Sensitivity. The color and absorption spectra of TDPP/ NDPP at different concentrations of Hg2+ ions were recorded to assess the sensitivity of the two probes. Both exhibited a gradual color change from yellow to blue in the presence of Hg2+, and the detection limit for Hg2+ was approximately 3 μM by naked eyes (Figure 5). Therefore, this approach may work for the colorimetric detection of Hg2+ in aqueous media near pollution sources. The TDPP and NDPP exhibited characteristic maximum absorption at 523 and 522 nm. Upon the addition of increasing Hg2+ concentrations (0−1 equiv), the F


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Analytical Chemistry process, the NDPP nanoparticle is a potential fluorescent probe for monitoring trace amounts of Hg2+ in water. Apparently, compared with raw TDPP, amphiphilic NDPP showed a better LOD for Hg2+ and good linear relation between Hg2+ concentration and fluorescence intensity, without a sharp change in fluorescence spectrum. The possible reason is that the TDPP core and TDPP−Hg complexation are effectively controlled in the inner portion of the NDPP nanoparticles in water. That is to say, the aggregation of TDPP in one nanoparticle will not influence another. Thus, the fluorescence quenching of NDPP is mild, which benefits sensing. However, in a monomeric TDPP-based sensing system, the formation of TDPP−Hg complexes can produce a gathering center for all rigid monomeric TDPP and TDPP− Hg complexes in the whole solution. When the amount of these gathering centers reaches a certain value, large-scale assembling behavior occurs, thereby inducing dramatic changes in the absorption and fluorescence spectrum, as shown in Figures 5c and 6a. Selectivity. Good selectivity is a critical feature for a desired probe. The effects of Ag+, Pb2+, Cu2+, Cr3+, Mn2+, Fe3+, Co2+, Ni2+, Cd2+, and Al3+ were investigated in a competitive experiment (Figures 7 and S10). Of these elements, only excess Ag+ (10-fold excess equiv) caused a slight change in the absorption and fluorescent spectrum of TDPP. By contrast, NDPP showed high selectivity toward Hg2+ (1.0 equiv) and exhibited negligible changes with all the other metal ions tested (10 and 100 equiv). In addition, a multimetal system also has little effect on the determination of Hg2+ in water using NDPP nanoparticles (Figure S11). The fluoride ion has been reported to react with DPP molecules containing primary amines.47 Thus, the behaviors of two DPP dyes with both inorganic and organic fluorides were studied. Results show that fluoride ions (1 mM) caused no noticeable changes in DPP dyes in aqueous media (Figure S12). Simultaneously, other common anions (Cl−, Br−, I−, CH3COO−, HCO3−, NO3−, H2PO4−, HPO42−, and SO42−) also have a negligible effect, even at high concentration (1 mM; Figure S13). Results suggest that selfassembled nanoparticles of NDPP are potential nanoprobes with high selectivity for Hg2+, as observed by both eye and fluorescence examinations. Adaptability. The effect of pH on detection was investigated, as DPP dyes with lactam moieties are sensitive to pH changes. The DPP responded to Hg2+ from pH 7.0 to 10.0, with fluorescent intensity varying by more than 50%; changes from pH 2.0 to 6.0 were negligible (Figure S14). One possible explanation is that the N atom of the lactam structure undergoes protonation and loses the lone pair electron for Hg2+ coordination in an acidic medium. By contrast, the amide structure is deprotonated and can easily react with Hg2+ in a basic medium. Thus, a DPP solution with neutral to mild basic conditions (pH value 7 to 10) benefits Hg2+ sensing. The effect of temperature was also investigated, as it significantly influences the aggregation behavior of DPP dyes (Figure S14). An obvious decrease in absorption intensity occurred at 580 nm when the temperature was increased from 10 to 70 °C in CH3OH/H2O (1:1, v/v; Figure S14). Such a result implies a decrease in the number of aggregated molecules at high temperature. Atmospheric temperature can maintain the interaction between Hg2+ and DPP dyes. In addition, the effect of high ionic strength was also evaluated. We found that NDPP could recognize Hg2+ in artificial neutral water, including Na+ (0.2 M), Ca2+ (0.1 M), Mg2+ (0.1 M), Cl−

(0.4 M), SO42− (0.1 M), but cost a long time (more than 2 h). When the pH was adjusted to 8, the recognition process becomes more effective (Figure S15). It means that NDPP is possible to work in weakly alkaline seawater. To evaluate the applicability of this method to real samples, river water (sampled directly from Xiang River, Changsha, and simply filtered) was spiked with Hg2+ and tested in the assay. As shown in Table 1, the results obtained for river water samples show good recovery values, which confirmed that the proposed nanoprobe was applicable for practical Hg2+ detection. Table 1. Recovery Study of Spiked Hg2+ in River Water with Proposed Sensing System samples

spiked (nM)

ICP-MS (nM)

NDPPa (nM)

recovery (%)

1 2 3

0 50 200

47 196

46.6 ± 0.7 185.5 ± 0.8

99.1 94.6

a

Mean value of three determinations ± standard deviation.

Hg2+ Ion Adsorption. Hg-triggered aggregation changes the optical properties of DPP dyes and causes the formation of large tight particles. The particles were filtered and analyzed via SEM. As shown in elemental mappings (Figure 8), mercury was

Figure 8. SEM images of the precipitates filtrated from (a) TDPP and (c) NDPP solution with 1.0 equiv of Hg2+. Panels (b) and (d) are relative elemental mappings of Hg (blue) from (a) and (c).

distributed uniformly across the precipitates. This observation implies that DPP dyes can detect Hg2+ and sequester the ions from the environment. Indeed, with suitable modifications, the DPP dyes may be grafted onto the surface of low-cost and ecofriendly nanomaterials or be used in gel systems to eliminate Hg2+ pollution. One advantage is that the removal of Hg2+ ions will be a visual process accompanied by an obvious color change. NDPP−Hg Nanoparticles for Cys Sensing. Cys is an important amino acid among the 20 different amino acids found in nature, as the compound is the only one containing a sulfhydryl. Our team has reported the electrochemical method for detection of Cys, based on sulfhydryl-Au reaction.48 Sulfhydryl also shows ultra-affinity to mercury because of stronger Hg−S coordination. Thus, NDPP−Hg nanoparticles may be used for Cys sensing because of Cys-induced disaggregation of TDPP units. To preliminarily evaluate this possibility, various concentrations of Cys (0−20 μM) were added into NDPP−Hg nanoparticles prepared in water by G



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mixing NDPP (10 μM) and Hg2+ (10 μM). Finally, the aggregation-evoked absorption band declined significantly (Figure S16), indicating the broken complexation of NDPP− Hg and tight molecular stacking. Given the good biocompatibility49 and fluorescence characteristic50 of DPP dyes, NDPP fluorescent nanoparticles is a potential tool for sensing of Cys in vivo.

CONCLUSIONS In summary, we studied the visual and fluorescent detection of Hg2+ by two DPP-based probes, namely, monomeric TDPP and NDPP-based loose micelle-like ONPs. Raw commercial dye TDPP possessing lactam-NH moiety can recognize Hg2+ through TDPP−Hg−TDPP complexation and molecular aggregation. NDPP, an amphiphilic TDPP, worked more effectively because of the formation of surfactant-free micellelike ONPs in water. NDPP ONPs are easier to fabricate than most surfactant-essential micelle-based ONPs. These fluorescent ONPs displayed good solubility in water and high selectivity toward Hg2+ over other competitive metal ions and anions. A wide linear response range of 0−4 μM and an estimated limit of detection of 11 nM were obtained. Our study demonstrated that surfactant-free micelle NDPP OPNs is a facile tool for sensing and adsorption of Hg2+. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b04258. Experimental data of XPS, IR, UV−vis, and fluorescent characterization for mechanism involved in TDPP-based system, and possible interference, application of such systems. The characterization for TDPP aggregation (Figures S1−3); the characterization of TDPP−Hg complexation and its effect to UV−vis absorption (Figures S4−6); the linear relationship between fluorescence intensity and Hg2+ concentration (Figure S9); The effects of common metal ions, anions, PH, cysteine, and temperature (Figures S10−15); The potential application of NDPP−Hg nanoparticles in sensing cysteine (Figure S16; PDF).

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhengchun Liu: 0000-0002-2897-8663 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported financially by the National Natural Science Foundation of China (Grant No. 61371042), the Central South University Faculty Research Fund (Grant No. 2013JSJJ060), the State Key Laboratory of Luminescent Materials and Devices at South China University ofTechnology (2013-SKLMD-08), and the Open-End Fund for the Valuable and Precision Instruments of Central South University. H


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