Solvatochromic fluorescent carbon dots as optic noses for sensing volatile organic compounds

Abstract

Amphiphilic carbon dots (CDs) with strong solvatochromism were synthesized via solvothermal method. The as-synthesized CDs demonstrate outstanding photoluminescence properties consisting of a high fluorescence quantum yield along with tunable photoluminescence (from blue to orange emission), which make them an ideal candidate for the detection of volatile organic compounds.

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With the rapid growth in civilization and economy, a strengthening emphasis has been put on environmental monitoring and human security which are adversely affected by serious environmental pollutions. Volatile organic compounds (VOCs) are low boiling organic solvents which are widely used in industries, agriculture and daily life, so exposure to VOCs is unavoidable. However, many VOCs can be toxic or carcinogenic even at relatively low concentrations. Therefore, real-time, sensitive detection and identification of VOCs are critically important to human health and environmental safety. Up to date, there have been numerous conventional methods for the detection of VOCs, including gas chromatography mass spectroscopy,¹ acoustic wave microsensor arrays,² wireless sensors³ and colorimetric detectors.⁴ However, most of these methods are not so convenient due to the requirement of expensive instruments as well as considerable expertise to operate. Therefore, there is a strong motivation to develop simpler, more convenient and efficient approaches for the detection of VOCs. Recently, fluorescent-based methods have been investigated for the detection of VOCs because of their advantages such as easy operation, high sensitivity, and fast response over other methods.^5,6 Nevertheless, the procedures for preparing these fluorescent sensors are laborious and time-consuming. As a consequence, the development of simple and reliable approaches for highly specific and rapid detection of VOCs is under active research.

Carbon dots (CDs) are luminescent carbon nanoparticles discovered by Scrivens et al. in 2004.⁷ Compared with conventional semiconductor quantum dots and organic dyes, CDs possess exceptional advantages such as low cost, easy modification, benign biocompatibility, high photostability as well as water solubility. These superior features make CDs arouse enormous attention due to their potential applications in diverse areas,^8–12 especially in sensors.¹³ CDs have been widely used for the detection of positive ions (such as Fe³⁺,¹⁴ Hg²⁺ (ref. 15) and Cu²⁺ (ref. 16) etc.), anions (Cr(VI),¹⁷ PO₄³⁻ (ref. 18) and F⁻ (ref. 19)) and biological molecules (glucose,²⁰ hydrogen peroxide,²¹ human α-fetoprotein²² and hyaluronidase²³) by monitoring the changes of their fluorescence. However, little work has been devoted to the development of CDs-based nanosensors for VOCs detection.

In pursuing new sensing materials, we have been attracted by the effect of solvatochromism, which represents a phenomenon where the absorption and emission spectra have a strong dependence with the solvent polarity. On the basis of the unique solvent dependence, we anticipate that the materials with solvatochromic effect can be used as a new type of sensor to probe VOCs rapidly and sensitively.

In the present work, amphiphilic CDs with strong solvatochromism were explored for VOCs detection. CDs were synthesized from (Z)-4-(2-cyano-2-(4′-(diphenylamino)-[1,1′-biphenyl]-4-yl)-vinyl)benzonitrile (pCN-TPA, Scheme 1) and PEG_2k (PEG with molecular weight of 2000). As-prepared CDs exhibit high solubility in both apolar (such as dichloromethane, DCM; ethylacetate, EA and tetrahydrofuran, THF) and polar media (for example dimethylformamide, DMF; acetonitrile, CAN; methanol, MeOH; ethanol, EtOH; acetone, DMK and water). Moreover, CDs demonstrate high fluorescence quantum yield (Q_f) in apolar (DCM, 30.7%) and polar (water, 19.4%) media as well as tunable photoluminescence (from blue, green, pink to orange emission), which endow them as ideal fluorescent sensors for the detection of VOCs. The most important is that an efficient test paper was prepared conveniently by casting CDs solution on the strip paper. This kind of solid sensor can act as an optic nose and sniff out VOCs instantaneously by changing fluorescence colors. Given the broad range of VOCs that can be detected and the high sensitivity of the sensor to these chemicals, this work highlights the great potential applications of CDs for the on-site detection of VOCs without any sophisticated processes or devices.

Scheme 1 Synthetic route of CDs. The inset is digital image of CDs solid under UV lamp with 365 nm illumination.

The precursor molecule, pCN-TPA is an intramolecular charge-transfer (ICT) fluorophores, which was synthesized according to the literature.²⁴ The UV-Vis absorption and photoluminescence (PL) properties of pCN-TPA were investigated in eight solvents. As shown in Fig. 1A and Table S1,† the dilute solution of pCN-TPA in eight different solvents exhibits similar maximum absorption bands at 383–395 nm. While pCN-TPA exhibits diverse PL behaviors in organic solvents (Fig. 1C and E), pCN-TPA emits very weak blue and green luminescence in strong polar organic solvents such as DMF (λ_em = 457 nm, Φ_f = 2.5%), MeOH (λ_em = 445 nm, Φ_f = 2.0%), EtOH (λ_em = 445 nm, Φ_f = 2.7%), DMK (λ_em = 459 nm, Φ_f = 2.7%) and ACN (λ_em = 504 nm, Φ_f = 2.6%). While the fluorescence color changes to orange in medium polar organic solvents (Fig. 1E) with higher Φ_f such as THF (λ_em = 630 nm, Φ_f = 7.6%), EA (λ_em = 624 nm, Φ_f = 22.0%) and DCM (λ_em = 630 nm, Φ_f = 10.1%). By contrast, the luminescence of pCN-TPA in solid state (Table S1,† Φ_f = 34.4%) is much higher than that in solution due to the phenomenon of aggregation-induced emission (AIE).

Fig. 1 The UV-Vis spectra of pCN-TPA (A) and CDs (B) in different solvents. The photoluminescence (PL) spectra of pCN-TPA (C) and CDs (D) in different solvents, λ_ex = 365 nm. Digital images of CDs (E) and pCN-TPA (F) in different solvents taken under UV lamp with 365 nm illumination.

Since pCN-TPA is very hydrophobic and has a very low solubility in aqueous solutions, which severely restricts the scope of application. In this work, a solvothermal technique was developed by trapping pCN-TPA into the core of PEG_2k to make fluorescent CDs. The synthetic route of CDs is shown in Scheme 1, briefly, the mixture of pCN-TPA (1.5 mg) and PEG_2k (200 mg) was dissolved in 10 mL of ethanol, and then the solution was transferred into a Teflon lined stainless autoclave. The sealed autoclave was heated to 100 °C and kept for 2 h. After cooling down, the crude product was purified by dialysis against deionized water and the final product was freeze-dried to yellow solid. The pCN-TPA loading content and loading encapsulation efficiency of CDs was determined to be 0.57 wt% and 77.0%, respectively, according to the characteristic absorption band of pCN-TPA at 394 nm in ethanol (see ESI, Fig. S1†). The solid of CDs emits a yellow fluorescence (Scheme 1, the inset) under UV illumination with high Φ_f of 36.3%, which is higher than that of pCN-TPA solid (34.4%).

The morphology and sizes of the as-synthesized CDs were characterized by transmission electron microscope (TEM), as shown in Fig. 2A, CDs are well monodispersed in water as a spherical shape with an average size of 3.24 ± 1.12 nm. As shown in Fig. S2,† the CDs could well disperse in water with concentration ranging from 0.5 to 5 mg mL⁻¹. Furthermore, 5 mg mL⁻¹ of CDs can be dispersible in common organic solvent. Fig. 2B displays ¹H NMR spectra of pCN-TPA and CDs in CDCl₃, since the content of pCN-TPA in CDs is as low as 0.57%, the resonances corresponding to pCN-TPA of CDs are very weak, only faint peaks can be seen from the enlarged ¹H NMR spectrum of CDs. On the contrary, a very strong peak (3.64 ppm) attributing to –CH₂ groups of PEG_2k is observed in ¹H NMR spectrum of CDs, indicating that there are lots of PEG_2k on the surface of CDs. It can be concluded from the FTIR spectra of pCN-TPA and CDs (Fig. 2C) that there are many characteristic peaks ascribing to –CH₂ (2886, 1469, 1343, 1272 and 841 cm⁻¹) and –O– groups (1110 and 963 cm⁻¹) of PEG_2k in CDs, while no features of pCN-TPA were observed in the FTIR spectrum of CDs, further confirming the very low content of pCN-TPA in CDs. X-ray photoelectron spectroscopy (XPS) was carried out to explore the composition of CDs as shown in Fig. 2D, two main peaks at 285.6 and 532.0 eV are assigned to C1s and O1s respectively. The high resolution C1s spectrum (Fig. 2E) can be well fitted into three peaks at 284.5, 285.7 and 288.4 eV, which correspond to C=C/C–C, C–OH and –O– species, respectively. The high resolution O1s spectrum (Fig. 2F) reveals the presence of C–OH (531.6 eV) and –O– (532.8 eV) groups. The stability of CDs in different condition was monitored by UV-Vis absorbance and fluorescence spectra. As shown in Fig. S3,† the CDs indicate good stability under irradiation and in different pH of solutions.

Fig. 2 (A) TEM image of CDs dispersed on a copper grid. The inset is the size distribution of CDs determined from TEM image analysis. (B) ¹H NMR spectra of pCN-TPA and CDs in CDCl₃, the inset displays the enlarged ¹H NMR spectrum of CDs in the range of 8.5 to 6.0 ppm. (C) FTIR spectra of pCN-TPA and CDs in the solid state. (D) Total XPS spectrum, (E) high resolution C1s and (F) O1s XPS spectra of CDs.

Compared with pCN-TPA, CDs are amphiphilic and highly soluble in apolar and polar solvents. The absorption spectra of CDs in the eight organic solvents are red-shifted more or less compared with the corresponding spectra of pCN-TPA, respectively (Fig. 1B). The only exception is the absorption behavior of CDs in water, which has a characteristic broad peak centered at 428 nm. It's worth mentioning that CDs not only exhibit multitudinous luminescence behaviors in the nine solvents (Fig. 1D) but also possess typical excitation-dependent feature in each solvent which is as similar with the previously reported carbon nanoparticles (Fig. S4†). CDs can emit pink (in DCM, EA and THF), orange (in water), green (in ACN) and blue light (in DMK, EtOH, MeOH and DMF) under the illumination of 365 nm (Fig. 1F). This solvatochromic behavior is very helpful for developing high sensitive VOCs sensors that exhibit changes in their fluorescent signals instantaneously when exposed to different solvents. In addition, the Φ_f of CDs (Table S1†) was dramatically enhanced by 1.7–7.7 fold compared to that of pCN-TPA due to the phenomenon of AIE (Fig. 1D and F and Table S1†).

The paper-based sensor strips were prepared by dropping the solution of CDs on the strips and air-dried. CDs on the strip emit bright yellow color under the UV illumination (365 nm) (Fig. 3A, left), then the fluorimetric response of the CDs sensor strip in the presence of MeOH vapor was studied. As displayed in Fig. 3A, the emission color of CDs immobilized strip turned immediately from orange to blue after exposed to MeOH vapor for 30 s at room temperature. It is worth noting that the orange to blue color transition is reversible, the emission color of CDs reverted back to the orange-colored state upon removal of the solvent vapor. The colorimetric response of CDs sensor toward other organic solvents vapor were also investigated and shown in Fig. 3B, EA and THF do not induce any observable color change to the CDs-based paper sensor CDs sensor. An orange to blue color transition was observed when the paper sensor strip was exposed to DCM, MeOH, ACN and DMF, and the orange colored image was changed to bluish orange upon exposure to EtOH and DMK. All of the fluorimetric transitions were observed to be reversible without losing the color intensity. In the sensing cycling/reversibility tests, all of the solvent samples could exhibit reversible qualitative solvate-optical sensing behaviors. These results demonstrated the excellent properties of this kind of CDs sensor strips such as high sensitivity, fast response and practicability.

Fig. 3 (A) The reversible emission color changes of CDs strip upon desolvation and solvation under the UV lamp (365 nm). (B) Photographs of the paper sensor strip printed with CDs after exposed to the vapor of solvents taken under the UV lamp (365 nm).

Conclusions

In summary, we report herein amphiphilic CDs with strong solvatochromism. The photoluminescence differences of CDs in apolar and polar solvents result in distinguishable fluorescence changes upon exposure to VOCs. The paper sensors prepared conveniently can distinguish VOCs in 30 seconds by the naked eyes. This work highlights the great potential of CDs-based sensors for a fast and precise discrimination of VOCs.

Acknowledgements

This project was supported by National Natural Science Foundation of China (No. 51522307), the Jilin Province Science and Technology Research Project (No. 20160101292JC) and the Research Project of Science and Technology of the Education Department of Jilin Province during the 13th Five-year Plan Period (No. 2016322).

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Footnote

† Electronic supplementary information (ESI) available: Materials, experimental methods and other additional data. See DOI: 10.1039/c6ra16055g

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