Thenoyltrifluoroacetone (TTA)-Carbon Dot/Aerogel Fluorescent Sensor for Lanthanide and Actinide Ions.

Contamination of groundwater with radioactive substances comprising actinides and lanthanides is a significant environmental hazard and thus the development of selective, sensitive, and easy-to-apply sensors for water-soluble actinide and lanthanide ions is highly sought. We constructed a new selective fluorescent sensor for UO2 2+, Sm3+, and Eu3+ based on a carbon dot (C-dot)-aerogel hybrid prepared through in situ carbonization of 2-thenoyltrifluoroacetone (TTA), a high-affinity heavy metal chelator. The TTA-C-dot-aerogel enabled the detection of UO2 2+ ions, which induced a significant red fluorescence shift, whereas Eu3+ and particularly Sm3+ ions gave rise to pronounced fluorescence quenching. Importantly, the lanthanide/actinide ion-selective TTA-C-dots could be synthesized only in situ inside the aerogel pores, indicating the crucial role of the aerogel host matrix both in enabling the formation of the C-dots and in promoting the adsorption and interactions of the lanthanide and actinide metal ions with the embedded C-dots.

Introduction

Varied industries and utilities such as coal and phosphate mines, fossil fuel power plants, and others generate substantial amounts of natural albeit hazardous toxic metal ions in soil and groundwater.[1] Environmental contamination with radioactive species such as actinide ion UO22+ and long half-life lanthanide fission products such as 151Sm3+ might also occur through intentional or accidental leakage (e.g., the nuclear accidents in Chernobyl and Fukushima). Human exposure to these ions occurs through the oral, dermal, wound, or respiratory routes, significantly increasing the risks of health hazards, including carcinogenesis, tissue fibrosis, and pneumoconiosis.[2−4] Accordingly, the development of sensors that can report on the presence of lanthanide and actinide ions is critical for alerting and implementation of strategies for the remediation of contaminated materials.[5,6]

Varied methods have been developed for the detection of lanthanides and actinides, including potentiometric membrane sensors,[7] sensing platforms based on colorimetric gold nanoparticles,[8] fluorescent molecular dyes,[9,10] conjugated polymer sensors,[11] and so on. Fluorescent sensors, in particular, exhibit practical advantages for metal ion detection in terms of sensitivity, selectivity, easy operation, low cost, and availability of multiple sensing parameters.[5,12] However, although numerous fluorescent sensors for metal ions have been reported over the past several years,[13−15] sensitive and selective fluorescence sensors for the detection of lanthanide and actinide metal ions are scarce. Specifically, significant practical limitations of lanthanide and actinide sensors have been encountered, including elaborate synthesis schemes, high cost of devices, and insufficient selectivity and/or sensitivity.[12,16]

Carbon dots (C-dots) constitute a family of fluorescent carbonaceous nanoparticles (<10 nm) and have attracted significant interest because of their unique structural and photophysical properties.[17−19] C-dots can be synthesized from inexpensive and readily available reagents using simple carbonization procedures.[20−22] Importantly, C-dots exhibit a broad range of excitation-dependent emission spectra, which are sensitive to the local environments of the dots, making possible the diverse sensing applications. In particular, metal ion sensing with C-dots has been reported.[23−25] An important aspect of C-dot chemistry in the context of sensing applications is the observation that these nanoparticles retain “structural memory” of the carbonaceous precursors, effectively endowing the dots with molecular recognition capabilities on the basis of functional units of the precursor molecules.[21] This remarkable feature has been employed for designing varied C-dot-based sensing modalities.[21,26,27]

Here, we report the construction of a new lanthanide and actinide ion sensor comprising silica aerogel embedding C-dots prepared from 2-thenoyltrifluoroacetone (abbreviated TTA) as the carbonaceous building block. TTA, a β-diketone ligand (Scheme ), has been employed as a scavenger for varied metal ions through chelating, forming relatively stable, insoluble complexes in aqueous solutions and organic solvents.[28,29] TTA has been employed as a chelator for lanthanide and actinide ions.[30] In particular, the ligand facilitated extraction of the ions in highly acidic solutions.[31] TTA has been also used as a conduit for sensing applications; low uranium concentrations could be detected, for example, via the luminescence properties of uranyl–TTA complexes.[32]

Scheme 1

Structure of the 2-Thenoyltrifluoroacetone (TTA) Ligand

Importantly, the TTA-based C-dots were synthesized directly within the pores of an aerogel framework. Aerogels, among the lowest density solids known, constitute excellent materials for sensing applications because of their porous structures and very high internal surface area available for the adsorption of guest molecules.[33,34] Varied types of aerogels have been synthesized, comprising silicon, carbon, metal ions, organic polymers, and so on.[34−38] Hydrophobic silica aerogels, in particular, have been used as insulation materials in the aerospace industry,[39] in the sorption of miscible organic solvents in water,[40] and as sensors for varied analytes both in solution and in the gas phase.[41,42]

The new hybrid aerogel, encapsulating in situ TTA-generated C-dots, is capable of reporting on specific lanthanide and actinide ions in water. We show that the fluorescence of the aerogel-embedded solid-phase TTA–C-dots underwent ion-specific quenching or distinct red shifts. The TTA–C-dot–aerogel construct is resilient, easy to produce in large quantities, and could be effectively used as a platform for the detection and speciation of lanthanide and actinide metal ions.

Results and Discussion

Synthesis and Characterization of the TTA–C-Dot–Aerogel

Figure illustrates the simple in situ TTA–C-dot–aerogel fabrication method. The aerogel host matrix was prepared through high-temperature silica annealing in the presence of pressurized nitrogen gas.[42] After aerogel synthesis and drying, the TTA precursor was infiltrated into the aerogel pores through incubation in a diethyl ether–water mixture and heating to generate the aerogel-embedded C-dots. The TTA–C-dot–aerogel consisted of easy-to-handle coarse powder, and it was resilient and stable at room temperature for long time periods (months).

Figure 1

Synthesis of the TTA–carbon-dot–aerogel. The TTA precursor was initially incubated with the aerogel matrix. Subsequent carbonization resulted in the formation of carbon dots inside the aerogel pores. The red dots represent the graphitic carbon cores of the C-dots, whereas residual TTA was retained on the surface of C-dots.

Figure presents the spectroscopic and analytical characterizations of the TTA–C-dot/aerogel hybrid, both confirming the formation of C-dots within the aerogel pores and retaining the porous aerogel framework. Typical excitation-dependent emission spectra were recorded for C-dots inside the aerogel host (Figure A). Notably, significantly lower fluorescence emission, exhibiting different shifts, was acquired when the C-dots were hydrothermally synthesized from TTA in aqueous solution in the absence of aerogel (Figure S1). This observation indicates significant aggregation of TTA–C-dots in solution and concomitant quenching of C-dots fluorescence, attesting to the crucial role of the aerogel matrix in C-dot formation and stabilization.

Figure 2

Characterization of the TTA–carbon-dot–aerogel. (A) Excitation-dependent emission spectra of the C-dots in C-dot–aerogel. (B) High-resolution transmission electron microscopy (HRTEM) images of C-dots extracted from the aerogel. The crystal planes of the C-dot graphitic core are apparent. (C) Brunauer–Emmett–Teller (BET) analysis of the TTA–C-dot–aerogel. (i) N2 adsorption–desorption isotherms. (ii) Pore size distribution curve indicating an average pore size of 4.8 nm.

The high-resolution transmission electron microscopy (HRTEM) images of the TTA–C-dots extracted from the aerogel matrix underscore the nanoscale dimensionality and crystalline nature of the carbon nanoparticles (Figure B). The HRTEM image shown in Figure B reveals a relatively uniform size distribution of the TTA–C-dots; C-dot diameters of 2.1 ± 0.5 nm were determined through the examination of several TEM images (Figure S2). The HRTEM image shown in Figure B (right) underscores the crystalline graphitic structure of the carbon cores of the C-dots, displaying the in-plane lattice spacing of 0.215 nm corresponding to the [110] plane of graphite.[21] X-ray photoelectron spectroscopy (XPS) analysis further confirmed the structural integrity of the TTA–C-dots (Figure S3).

We further examined the effect of C-dot formation on the properties of the aerogel matrix, particularly its overall porosity, a critical parameter in the context of sensing applicability. Specifically, Brunauer–Emmett–Teller (BET) analysis of the TTA–C-dot–aerogel, shown in Figure C, indicates a relatively high specific surface area of 450 m2/g (Figure C,i), average pore diameter of 4.8 nm (Figure C,ii), and pore volume of 0.51 cm3/g. These values are similar to those of pristine aerogels (not containing embedded C-dots), confirming that the open porous structure of the aerogel was retained after the in situ synthesis process.

Fluorescence Sensing of Lanthanide and Actinide Metal Ions

Application of the TTA–C-dot–aerogel hybrid for the detection of lanthanide and actinide ions is depicted in Figures and 4. Figure A presents the fluorescence emission spectra recorded following the addition of aqueous solutions of UO22+, Eu3+, or Sm3+ ions to the TTA–C-dot/aerogel. The fluorescence spectra in Figure A reveal two distinct effects. Specifically, a significant red shift of the emission peak, from 445 to 480 nm (excitation at 400 nm), occurred upon addition of UO22+ to the TTA–C-dot–aerogel (Figure A). Importantly, to the best of our knowledge, this is the first report on ion-induced red shift of C-dot fluorescence. This shift is likely due to changes in electron density at the C-dots’ surface because of coordination of the UO22+/TTA chelating complex.[43] Red fluorescence shifts in case of conjugated polymers which form chelate with metal ions have been reported and ascribed to the modulation of conjugation network and surface electron densities.[43] The other effect highlighted in Figure A is fluorescence quenching induced upon addition of Eu3+ and Sm3+ ions to the TTA–C-dot/aerogel; the quenching effect was particularly apparent upon excitation at 350 nm (Figure A, right panel).

Figure 3

Effect of lanthanide and actinide ions on the fluorescence properties of C-dot–aerogels. (A, B) Fluorescence emission spectra (excitation wavelengths indicated) recorded following addition of the indicated ions: (A) TTA–C-dot–aerogel and (B) glucose derivative-C-dot–aerogel. (C) Confocal fluorescence microscopy images (exc. 405 nm and emission filter EM 445/60) of the TTA–C-dot–aerogel before and after the addition of Sm3+ ions (concentration 100 ppm).

Figure 4

Fluorescence modulation of the TTA–C-dot–aerogel upon addition of metal ions. Concentration-dependent (A) shifts of the fluorescence peak (λex 400 nm/λem 445 nm) and (B) intensity (λex 350 nm/λem 405 nm). Comparison of the (C) fluorescence shifts (λex 400 nm/λem 445 nm) and (D) fluorescence intensities (λex 350 nm/λem 405 nm) upon addition of different metal ions: (a) control TTA–C-dot/aerogel, (b) Sm3+, (c) Eu3+, (d) UO22+, (e) Ce3+, (f) Nd3+, (g) Gd3+, (h) Cd2+, and (i) Pb2+. All ions were at a concentration of 100 ppm.

To investigate whether the significant ion-induced fluorescence modulation of the C-dot–aerogel in Figure A (shifts of emission peaks or fluorescence quenching) was due to the use of TTA as the carbonaceous precursor, we further tested a C-dot–aerogel hybrid in which the embedded C-dots were synthesized in situ from a different precursor, 6-O-(O-O′-di-lauroyl-tartaryl)-d-glucose (Figure B). Indeed, Figure B demonstrates that neither emission shifts nor fluorescence quenching was induced upon the addition of UO22+, Eu3+, or Sm3+ ions to the C-dot–aerogel not prepared from TTA as the carbon source. These results are significant, as they clearly underscore the central role of the TTA units in determining the sensing features of the new TTA–C-dot–aerogel hybrid. The TTA–C-dot–aerogel system can also be employed for sensing lanthanide ions through fluorescence microscopy (Figure C). The confocal fluorescence microscopy images in Figure C (exc. 405 nm) show that the initial bright fluorescence of the C-dot–aerogel particles was substantially quenched upon addition of Sm3+ ions. Notably, a direct relationship between Sm3+ concentration and the extent of fluorescence signal attenuation was clearly apparent (Figure S4).

Figure highlights the sensitivity and selectivity features of the new TTA–C-dot–aerogel hybrid. Figure A depicts the concentration-dependent shifts of the fluorescence emission (excitation 400 nm) upon addition of UO22+, Sm3+, or Eu3+. Notably, only UO22+ induced an experimentally significant fluorescence shift, recorded even in low (<10 ppm) concentrations (Figure A). The graph in Figure B, depicting the fluorescence quenching (excitation 350 nm, emission 405 nm) versus ion concentrations, confirms the direct relationship between metal ion concentrations and the extent of C-dots’ fluorescence quenching, indicating that adsorption of the ions within the aerogel pores accounted for the fluorescence quenching effect. Importantly, Figure B also indicates that the most pronounced fluorescence quenching of the C-dots’ fluorescence was induced by the Sm3+ ion, reflected in the detection threshold of 0.5 ppm. Lesser, albeit experimentally significant, fluorescence quenching was induced by UO22+ and Eu3+ ions at higher concentrations. Although metal-ion-induced quenching of C-dots’ fluorescence has been widely reported,[5,6]Figure B provides an extraordinary demonstration of selective lanthanide-induced fluorescence quenching via specifically tailored C-dots.

The bar diagrams in Figure C,D highlight the selectivity of the TTA–C-dot–aerogel platform. Significantly lesser fluorescence shifts (Figure C) or quenching (Figure D) was recorded upon addition of ions such as Cd2+, Pb2+, and Na+ to the TTA–C-dot–aerogel. The selectivity of the TTA–C-dot/aerogel hybrid for UO22+ (fluorescence shift) and Sm3+ and Eu3+ (fluorescence quenching) likely reflects the high affinity of TTA toward these ions.[44−46] The pronounced binding of the ions to the TTA units at the C-dot surface affects their surface energy states, likely accounting for metal-ion-induced quenching of C-dot fluorescence.[23] The TTA-C-dots synthesized inside the aerogel host was extracted and excitation dependent emission property was observed in Figure S5 which revealed the similar property like Figure A.

To further probe the effects of the lanthanide and actinide ions on the TTA–C-dots encapsulated within the aerogel matrix, we carried out Raman scattering experiments (Figure ). The Raman spectra in Figure corroborate the fluorescence data in Figures and 4, indicating significant ion-dependent modifications of the TTA–C-dots surface units. The Raman spectrum of the untreated TTA–C-dot–aerogel (Figure i) exhibits several vibrations ascribed to the TTA unit,[47] including two main peaks around 1418 cm–1 assigned to the symmetric C=C–C=C stretching of the thienyl group and a vibration band at 1522 cm–1 assigned to the C=C–C=O stretching.

Figure 5

Raman spectra of the TTA–C-dots–aerogel. Ion concentrations were 100 ppm.

The effects of Sm3+ (Figure ii) and UO22+ (Figure iii) on the Raman spectra were significant and attributed to metal-induced conformational changes of the residual TTA on the C-dots’ surface. Specifically, the main peak at 1417 cm–1 splits upon addition of Sm3+, giving rise to a shoulder at 1447 cm–1. Likewise, the Raman signal at 1526 cm–1 is more intense following the addition of either UO22+ or Sm3+ ions compared to that with the bare TTA–C-dot. Notably, the peak at 1610 cm–1, corresponding to the C=C–C=O unit coupled to the OH bending mode (enolate form),[48] disappeared upon incubation of the TTA–C-dot–aerogel with UO22+ (Figure iii). This spectral modification is ascribed to the interaction between the OH group and the uranyl ion. Furthermore, the weaker vibrational bands at 400–790 cm–1 in the presence of the two ions may be traced to covalent interactions between the TTA–C-dot and the metal ions.[49,50] Overall, the Raman scattering data provide structural evidence for both retention of the TTA units on the C-dots and participation of the TTA residues in the interactions with UO22+, Sm3+, and Eu3+. These interactions likely account for the significant fluorescence shifts and changes in intensities induced by the ions.

Conclusions

We present a new fluorescent sensor for lanthanide and actinide metal ions based on TTA-generated C-dots prepared in situ within the porous framework of silica aerogel. The structural and physical properties of both the aerogel matrix and embedded C-dots were retained following the synthesis procedure. Importantly, the aerogel framework was critical for the assembly of the fluorescent C-dots. The TTA–C-dot–aerogel hybrid constituted a selective and sensitive fluorescence sensor for UO22+, Sm3+, and Eu3+ ions, which gave rise to significant red shift of the C-dots’ fluorescence (in case of UO22+) or dramatic fluorescence quenching (Sm3+ and Eu3+). The TTA–C-dot–aerogel sensor exhibits notable practical advantages. Preparation of the hybrid C-dot/aerogel material is straightforward, using inexpensive and readily available reagents. The TTA–C-dot–aerogel can be stored as a powder for long time periods. Sensing experiments are easy to perform and can be carried out through either fluorescence spectroscopy or microscopy. Importantly, the selectivity and sensitivity of the C-dot–aerogel sensor are on par or better than those of most lanthanide/actinide sensors reported thus far. This work further underscores the versatility of C-dots/aerogel constructs as sensing platforms for diverse analytes.

Experimental Section

Materials

Tetraethylorthosilicate (TEOS), 2-thenoyltrifluoroacetone (TTA), d-(+)-glucose, sodium sulfate, pyridine, ammonium hydroxide, lead nitrate, and cadmium nitrate were purchased from Sigma-Aldrich. l-(+)-Tartaric acid and sodium hydroxide were purchased from Alfa-Aesar, England. Lauroyl chloride was purchased from TCI, Japan. Europium chloride hexahydrate and samarium chloride hexahydrate were purchased from Strem Chemicals Inc. Uranyl nitrate hexahydrate (UO2(NO3)2·6H2O) was purchased from Inorganic Ventures, Inc. Ethanol was purchased from J. T. Baker. Diethyl ether, ethyl acetate, and concentrated hydrochloric acid (HCl) were purchased from Bio Lab Ltd, Jerusalem, Israel.

Aerogel Synthesis

Wet silica gel was prepared according to previous reports.[17,33] Briefly, 5 mL of TEOS, 15 mL of anhydrous ethanol (EtOH), 5 mL of distilled water, and 5 μL of concentrated hydrochloric acid were mixed in a 100 mL flask and stirred in a 60 °C water bath for 90 min. Subsequently, 25 mL of ethanol, 13 mL of distilled water, and 15 μL of NH4OH were added to the solution and stirred for 30 min at the same temperature. The prepared wet silica gel was coated with parafilm before it was further dried and transferred into 200 mL of anhydrous ethanol and placed in a GCF1400 atmosphere furnace under an ultrapure N2 gas atmosphere. The outlet was subsequently closed while the ultrapure N2 was continuously passed into the autoclave and it reached 1 MPa. The temperature was first raised quickly from room temperature to 200 °C, increased slowly to 246 °C, followed by 260 °C for 3 h at 2 MPa N2 gas pressure. White silica aerogel was obtained after opening the autoclave.

In Situ Synthesis of TTA–C-Dot–Aerogel

Around 50 mg of the TTA precursor was mixed with 200 mg of aerogel in a glass vial, and 300 μL of diethyl ether and 500 μL of distilled water were added to the mixture. The suspension was then sonicated for 5 min and heated at 140 °C for 6 h. The synthesized C-dot–aerogel was purified by diethyl ether several times to remove C-dots that were not embedded within the aerogel.

TTA–C-Dot Synthesis in Aqueous Solution (without Aerogel)

Around 50 mg of the TTA precursor was dissolved in 300 μL of diethyl ether, and 500 μL of distilled water was added to the mixture. The suspension was then sonicated for 5 min and heated at 140 °C for 6 h.

In Situ Synthesis of Glucose Derivative-C-Dot–Aerogel

Carbon dot precursor 6-O-(O-O′-dilauroyl-tartaryl)-d-glucose was synthesized according to our published report.[17] Around 10 mg of the C-dot precursor was mixed with 100 mg of aerogel in a glass vial, and 300 μL of distilled water was added to the mixture. The suspension was then sonicated and heated at 125 °C for 2.5 h. The synthesized C-dots–aerogel was purified by CHCl3 several times to remove the unbound C-dots.

Ion Sensing

Metal ions were dissolved in distilled water at different concentrations (1–1000 ppm). Then, 50 μL of each ion was placed upon 50 mg of the C-dot–aerogel powder and incubated for 10 min. Fluorescence of the C-dot–aerogel slurry was recorded with different excitation wavelengths.

Instrumentation and Characterization

High-resolution transmission electron microscopy (HRTEM) experiments were carried out using TTA–C-dot–aerogel dissolved in chloroform for extraction of the carbon dots from the aerogel matrix. The HRTEM samples were prepared by placing a drop of solution on a graphene-coated copper grid and observed with a 200 kV JEOL JEM-2100F microscope (Japan). Fluorescence emission spectra of the C-dot–aerogel using different excitation wavelengths were recorded on a Varioskan plate reader. Confocal microscopy images were acquired on an Ultra View system (PerkinElmer Life Sciences, Waltham, MA) equipped with an Axiovert-200 M microscope (Zeiss, Oberkochen, Germany) and a Plan-Neofluar 63×/1.4 oil objective with 405 nm laser excitation wavelength. Raman spectra were recorded with a Horiba-Jobin-Yvon LabRam HR 800 micro-Raman system, equipped with a Synapse CCD detector. The excitation source was an argon laser (514.5 nm), with a power of 5 mW. The laser was focused with a 100× long-focal-length objective to a spot of about 1 μm. Measurements were taken with 600 g/mm grating and a confocal hole of 100 μm with a typical exposure time of 1 min.