HP-cyclodextrin modified sulphur quantum dots for the fluorescent cage sensing of p-NP by structural matching and PET: a new sensing approach†

Yining Chang^ab, Ran He^ab, Runqiu Wang^a, Yanli Wei*^a and Li Wang*^a
aInstitute of Environmental Science, Shanxi University, Taiyuan, 030031, China. E-mail: weiyanli@sxu.edu.cn; wangli@sxu.edu.cn
^bSchool of Chemistry and Chemical Engineering, Shanxi University, Taiyuan, 030006, China

First published on 7th August 2024

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Introduction

Nitrophenols are manufactured chemicals for the production of dyes, pesticides, wood preservatives, photographic chemicals and medicines.¹ During the production processes, the discharge of para-nitrophenol (p-NP) into the environment becomes inevitable through waste water or unexpected accidents. As a persistent environmental pollutant, the intake of p-NP can cause headache, cyanosis, corneal opacity or cataracts, and potential carcinogenicity.^2,3 Once deposited in soil, phytotoxicity poses a real threat to forestry and vegetation, so p-NP is listed as a harmful environmental pollutant by the Environmental Protection Agency of United States.^4,5 Moreover, p-NP is listed as a priority environmental pollutant because some studies have revealed that p-NP induces genotoxic effects on cells and causes respiratory complications.⁶ Therefore, the rapid, sensitive and efficient detection of nitrophenols has great significance for human health, food safety and environmental protection. So far, various analytical methods have been used for nitrophenol detection, such as spectrophotometry,⁷ capillary electrophoresis,⁸ electrochemistry⁹ and liquid chromatography.¹⁰ However, some obvious limitations exist, such as the complicated sample pretreatment, expensive equipment and long processing time. Apparently, fluorescence technology has advantages over other methods, including its low cost, easy operation, less time-consuming, and environmental benignity.^11,12 With the emergence of nanotechnology as a key technology for the engineered nanomaterials, various fluorescent sensors have been developed with metal–organic frameworks, quantum dots, metal nanoparticles, and nanosheets,^13,14 which have been applied for the detection of intracellular reactive oxygen species¹⁵ and the specific imaging of cancer cells.¹⁶

Sulphur quantum dots (SQDs) are a new member of non-metallic luminescent quantum dots that contain a large number of sulfate, sulfite, and polar functional groups of modifiers, offering them good water dispersion and biocompatibility.¹⁷ In addition, SQDs have been widely used in metal ions and biomolecular fluorescence detection, optoelectronic devices, and cell imaging because of their unique photoluminescence properties, good photostability, low toxicity, and bleaching resistance.^18,19 Of these, carbon quantum dots (CQDs),²⁰ graphene nanomaterials²¹ and metal organic frameworks²² have been used for the detection of nitrophenols, and even the visual monitoring of p-NP has been reported.²³ However, the most common fluorescence sensing mechanism was reported to be the inner filter effect (IFE) and/or static quenching.²⁴

Molecular structure matching is usually considered for the interaction between protein and small molecules, which occurs in the cavities or grooves from the structural protein.²⁵ Structural matching is considered a new approach for developing fluorescent cage sensing. As a class of naturally-occurring oligosaccharide, β-cyclodextrin (β-CD) consists of seven D-glucopyranose units that form a torus having a hydrophobic inner surface and a hydrophilic outer surface. Cyclodextrins have been used as a component of artificial enzymes and molecular cages.²⁶ The hydrophobic cavity causes the prearrangement of substrates, and leads to the selective exposure of affected groups involved in the reaction or interaction. Previous results have reported the increased regioselectivity of the products in the order of ortho-, meta-, and para-substituted substituents, which was attributed to the different binding abilities of aromatic substrates with the interior cavity of β-CD.^26,27 As compared with β-CD, 2-hydroxylpropyl-β-cyclodextrin (HP-β-CD) has better water solubility and better stability in alkaline solutions, and it was selected as the stabilizer to synthesize HP-SQDs for the fluorescent sensing of p-NP.

This study couples the luminescence feature of SQDs with the selectivity from the structural matching of HP-β-CD and nitrophenols to achieve the selective detection for p-NP over the other two isomers, and meticulous mechanistic studies were performed. HP-β-CD as surface passivator modified sulphur quantum dots (HP-SQDs) have good water solubility, high photostability and a blue-emission at around 440 nm, which render them as a good candidate for fluorescent sensing. This study uses HP-SQDs as the molecular recognition and fluorescent signal for the detection of p-NP. As shown in Scheme 1, the optimized structural matching and photoinduced electron transfer (PET) exist between p-NP and HP-SQDs. The sensitive detection of p-NP by HP-SQDs was confirmed from the calculation of free energy and inclusion constant between HP-β-CD and nitrophenol isomers. Finally, HP-SQDs were applied to the detection of real samples and demonstrated its potential sensing abilities.

Experimental

Materials and instruments

Sublimated sulphur powder was purchased from Beijing InnoChem Science & Technology Co., Ltd. HP-β-CD, p-NP, o-NP and m-NP were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Sodium hydroxide (NaOH) was purchased from Shanghai Macklin Biochemical Technology Co., Ltd. The steady state fluorescence spectra and time-resolved decay traces were recorded with the PTI QuantaMaster 400 (Horiba). The UV-visible absorption spectra were recorded using a Cary 8454 UV-visible spectrophotometer (Agilent). The infrared spectra were recorded with BRUKER Tensor II infrared spectrometer (Germany). X-ray photoelectron energy spectra (XPS) were recorded with the K-Alpha™ XPS System from Thermo Scientific™ (USA). The transmission electron microscopy (TEM) images were recorded using the US FEI Tecnai G2 F20 microscope.

Synthesis of HP-SQDs

HP-SQDs were prepared with reference to the reported procedure with some modifications.¹⁷ HP-β-CD (optimized amount: 2.0 g, 1.3 × 10⁻³ mol) was first dissolved in a round-bottom flask containing NaOH (2.0 mmol L⁻¹, 50 mL), and stirred at room temperature. Then, sublimated sulphur powder (1.4 g, 4.4 × 10⁻² mol) was added to the solution after HP-β-CD was completely dissolved. The solution was aerated with pure oxygen and continuously stirred at 85 °C for 48 h under optimized conditions (Fig. S1, ESI†). After the reaction was completed, the product was purified with a dialysis membrane (molecular weight: 2000 Da) and the deionized water was changed every 4 h. After 24 h dialysis and freeze-drying, the light-yellow powdery HP-SQDs were obtained, and refrigerated (4 °C) for subsequent experiments.

Detection of nitrophenol isomers

HP-SQDs as a fluorescent sensor were used for the detection of nitrophenol isomers at room temperature. A solution of 100 μL of HP-SQDs (10 mg mL⁻¹) was mixed separately with o-/m-/p-NP (5.0 mmol L⁻¹) at different volumes, and then the mixed solution was diluted with PBS buffer (pH = 8, 20 mmol L⁻¹) to a total volume of 2.0 mL and incubated for 5 min. With the excitation of 320 nm, the fluorescence spectra were scanned from 350 nm to 600 nm. Both excitation and emission slits were set at 1.92 nm (5 nm bandpass).

Molecular simulation

Theoretical calculations were conducted using Gaussian 16. The structures of HP-β-CD and nitrophenol isomers were initially optimized at the M062X/6-31G(d) level of theory using the Gaussian 16 package. The optimized molecular structures of the isolated molecules are shown in Fig. S12 (ESI†). These geometries were used to prepare the initial structure of the host–guest complexes.

The structures corresponding to various minima were further optimized on the M062X/6-31G(d) level of theory. The interaction energies (ΔE) for inclusion complexes were computed by supramolecular approach.

ΔE = E0complex − (E0guest + E0host)	(1)

where E⁰_complex, E⁰_guest and E⁰_host are the energies of the located structures of the inclusion complex, guest and host molecules, respectively. Generally, the weak interaction between the A and B molecules cannot be calculated simply by eqn (1). The basis set superposition error (BSSE) must be corrected, and eqn (1) is modified to eqn (2).

ΔE = E0complex − (E0guest + E0host) + E0BSSE	(2)

The DFT/M062X/6-31G(d) level of theory was applied, and the calculations were conducted in the Gaussian 16 program.

Verification with real samples

The application of the developed fluorescent sensor of HP-SQDs was conducted with 100 μL of HP-SQDs solutions (10 mg mL⁻¹) and the spiked water samples with nitrophenol isomers at different concentrations. Water samples were collected from Jinyang Lake (Taiyuan, Shanxi), industrial wastewater (Daily Life Chemical Industry Company, Jinzhong) and tap water. The water sample was first filtered through a 0.45-μm filter membrane, then spiked with o-NP to a final concentration of o-NP at 20, 75 and 125 μmol L⁻¹, respectively. The sensing solution of HP-SQDs was added to the spiked water sample, and the fluorescence spectrum was recorded under the optimized conditions. The recovery was calculated by the fluorescence intensity at 440 nm. The recovery of m-NP and p-NP was calculated following the same procedure, with the spiked m-NP at the concentration of 25, 75, and 125 μmol L⁻¹, and the spiked p-NP at the concentration of 10, 20 and 35 μmol L⁻¹. The industrial wastewater was spiked at 40, 60, 80 μmol L⁻¹. Six measurements were taken for the spiked o-/m-/p-NP at each concentration to calculate the average recovery rate.

Results and discussion

Characterization of HP-SQDs

The morphology and size distribution of HP-SQDs were characterized by TEM, as shown in Fig. 1(a)–(c). The prepared HP-SQDs were evenly distributed as a spherical structure with the average particle size of 1.9 ± 0.011 nm, with the lattice spacing of 0.209 nm. The FT-IR spectra of sublimated sulphur, HP-SQDs, and HP-β-CD are displayed from the top to the bottom in Fig. 1(d). The stretching and bending vibrations of –OH resulted in absorptions at 3416 cm⁻¹ and 1636 cm⁻¹, respectively.^28,29 The absorption at 2928 cm⁻¹ is from the stretching vibration of C–H.³⁰ Different from the sublimated sulphur, the formation of HP-SQDs resulted in two new absorption bands appearing at 1034 cm⁻¹ and 1155 cm⁻¹, which were attributed to the stretching vibration of C–O from HP-β-CD and the bending vibration of –OH.³¹ Therefore, the changes in the IR absorption indicate that HP-β-CD was anchored on the surface of SQDs. The full XPS spectrum of HP-SQDs is shown in Fig. 1(e). Three peaks appear at 164.80 eV, 285.11 eV, and 535.02 eV, indicating that the synthesized HP-SQDs have three elements of S, C, and O, respectively. As shown in Fig. 1(f), the high-resolution spectrum of sulphur elements was deconvoluted into six different peaks. The three peaks at 161.20 eV, 163.5 eV, and 164.2 eV were attributed to the characteristic peaks of the S[0], and the peaks at 166.30 eV, 167.08 eV, and 168.70 eV were assigned to the sulphur oxide species, existing either as SO₂²⁻ (2p^2/3), SO₂²⁻ (2p^1/2), or SO₃²⁻ (2p^2/3), or SO₃²⁻ (2p^1/2).³² Therefore, the XPS results revealed that the primary components of HP-SQDs are zero-valent sulphur, sulfite, as well as sulfonate groups on their surface. The crystal size of the synthesized SQDs is very small, about 1.9 ± 0.011 nm, and such small crystallites broaden the diffraction peaks (Fig. S2, ESI†). The variations in d-spacing caused by micro-strain may also cause the peak broadening.³³ The Raman spectra (Fig. S3, ESI†) of HP-SQDs were very similar to the Raman spectrum of sublimed sulphur. All characterization data confirmed the obtained product as SQDs.^34,35

Photophysical properties of HP-SQDs

The UV-visible absorption of HP-SQDs is shown in Fig. 2(a). There are two broad absorption bands at around 330 nm and 360 nm, which could arise from the direct bandgap transitions of elemental sulphur and polysulphur ions, with reference to previously reported results.^36,37 The absorption band at 212 nm possibly arises from the n–σ* transition because the heteroatoms (O, S) on the surface HP-SQDs have non-bounded long pair electrons.³⁸ All absorption transitions suggest the formation of HP-SQDs.

The fluorescence spectra of HP-SQDs at the excitation of 300–380 nm are displayed in Fig. 2(b), which is wavelength-dependent.³⁹ With the change of excitation wavelength, the emission maximum of HP-SQDs was redshifted from 434 nm to 479 nm. Along with the red shift, the fluorescence intensity first increased and reached the maximum at 320 nm excitation, then decreased again. The excitation-dependent emission reveals that the prepared HP-SQDs have a wide size gamut. The excitation spectrum was scanned with λ_em at 440 nm, as shown in Fig. 2(a), which indicated that the excitation at 320 nm generated the most intense fluorescence. With the excitation at 320 nm, the emission maximum was centred at 440 nm. The fluorescence quantum yield of HP-SQDs in water was determined as 0.030 at 340 nm excitation with quinine sulphate in 0.10 mol L⁻¹ H₂SO₄ as a reference.

The stability of quantum dots at different conditions will directly affect the analytical results, so the experimental condition was first optimized before the sample analysis. The photostability of HP-SQDs in water was investigated by varying the pH, solution temperature, and the addition of electrolyte. The fluorescence change of HP-SQDs was evaluated by the ratio of F/F₀ at different experimental conditions, where F₀ represents the original fluorescence intensity of HP-SQDs in water at room temperature, and F represents the fluorescence intensity of HP-SQDs under the various conditions. The fluorescence intensity of HP-SQDs increased progressively as the pH values increased, as shown in Fig. S4(a) (ESI†). In Fig. S4(b) (ESI†), the fluorescence intensity of HP-SQDs is relatively stable in the varied concentration of NaCl solution from 0.10 mol L⁻¹ to 1.0 mol L⁻¹, indicating that the fluorescence intensity of HP-SQDs was not affected by ionic strength. The results from Fig. S4(c) (ESI†) indicate that the variation of temperature also does not have an apparent effect on the fluorescence intensity of HP-SQDs at the temperature range of 10–60 °C. The emission intensity of HP-SQDs was almost constant for seven days after the preparation, as shown in Fig. S4(d) (ESI†), which indicates that the storage time at 4 °C does not have an observable effect on the stability of HP-SQDs. The results confirmed that the HP-SQDs have great stability.

Detection of HP-SQDs for nitrophenol isomers

To achieve the best results of HP-SQDs on the detection of nitrophenols, a series of optimizations were conducted, such as the concentration of SQDs, incubation time and temperature. First, the effect of the HP-SQDs dosage on the detection sensitivity was examined at the various concentrations of 0.25, 0.35, 0.50, 0.60, 0.75 mg mL⁻¹. From Fig. 3(a), the intensity ratio of the sensing system reached the highest at the HP-SQDs concentration of 0.50 mg mL⁻¹. Therefore, the optimized dosage of HP-SQDs was set at 0.50 mg mL⁻¹ for the rest of the experimental study. As shown in Fig. 3(b), HP-SQDs could effectively distinguish p-NP from the other two isomers under acidic conditions, but could not effectively distinguish between o-NP and m-NP. With the increased pH to values greater than 8.0, the HP-SQDs sensing system displayed an emission intensity ratio of F/F₀ for the three nitrophenol isomers. At pH = 8.0, the difference was more apparent than at other pH values. Therefore, the optimized sensing condition was set at pH = 8.0 for the following experiment. From Fig. 3(c), it is clear that the HP-SQDs can apparently distinguish the three nitrophenol isomers when the incubation time reaches 5 min, which suggests that the HP-SQDs could ensure a fast detection for nitrophenol.

All three nitrophenols can cause fluorescence quenching of HP-SQDs. As shown in Fig. 4(a), (c) and (e), with the successive addition of the three nitrophenol isomers, the fluorescence intensity of HP-SQDs at 440 nm gradually decreased. The emission maximum of HP-SQDs was also red-shifted to 456 nm with the addition of o-NP and red shifted to 476 nm with the addition of p-NP, but it remained almost unchanged with the addition of m-NP, as shown in Fig. S5 (ESI†). Therefore, the three nitrophenol isomers could be distinguished by the emission spectra.

At the concentration of o-NP ranging from 5.0 μmol L⁻¹ to 150 μmol L⁻¹, the plot of F/F₀ against the concentration (together with the linear fitting) are shown in Fig. 4(b), with the fitting equation as y = 5.830 × 10⁻³·x + 0.9652 and the correlation coefficient of R² = 0.998. The limit of detection (LOD = 3σ/k) was calculated as 0.79 μmol L⁻¹. In Fig. 4(d), at the concentrations of m-NP ranging from 0 to 175 μmol L⁻¹, the linear fitting for the plot of F/F₀ against the concentration is y = 8.030 × 10⁻³·x + 10104, with the correlation coefficient of R² = 0.987, and the LOD was calculated as 0.57 μmol L⁻¹. When the concentration of p-NP increased from 0 to 262.5 μmol L⁻¹, the corresponding linear fitting for the plot of F/F₀ against the concentration was as y = 1.866 × 10⁻²·x + 0.9821 with the correlation coefficient of R² = 0.991, and the LOD was calculated as 0.25 μmol L⁻¹ at the concentration range from 2.5 μmol L⁻¹ to 45 μmol L⁻¹, as shown in Fig. 4(f). Also, the most efficient quenching of HP-SQDs by p-NP became apparent when observed under the UV light (Fig. S6, ESI†). The results indicated that the sensing of p-NP by HP-SQDs has achieved an ideal result. As compared with other reported methods for the sensing of p-NP (Table S1, ESI†), the results indicated that the analytical performance towards p-NP from this method is comparable to previously reported studies.

Selectivity, specificity and stability of HP-SQDs

The selectivity of HP-SQDs was investigated against possible environmental interferents, such as cations (K⁺, Ca²⁺, Cu²⁺, Fe²⁺, Al³⁺), anions (ClO⁻, HCO₃⁻, NO₂⁻, C₂O₄²⁻, MoO₄²⁻, P₂O₇⁴⁻), and aromatic analogues nitrobenzene (NB), 4-nitrotoluene (p-NT), 2,4-dinitrotoluene (2,4-DNT), 4-aminobenzoic acid (p-ABA), 2,4,6-trinitromethylbenzene (TNT), 2,4,6-trinitrophenol (TNP), 4-cyanophenol (p-CP), 2-aminophenol (o-AP), 4-aminophenol (p-AP) based on the fluorescence intensity changes of HP-SQDs in the presence of these interferents in PBS buffer (pH = 8.0). As shown in Fig. 5, the effect of interferents (20 μmol L⁻¹) on the fluorescence quenching of HP-SQDs was studied, and the results indicated that these interferents did not have any apparent influence on the fluorescence intensity of HP-SQDs. The fluorescence from the mixture of HP-SQDs/p-NP and the interferents mentioned earlier were also measured to evaluate the specificity of HP-SQDs towards p-NP. As shown in Fig. 5, there was no apparent interference effect on HP-SQDs in the presence of nitrophenols. Therefore, the HP-SQDs have high selectivity and specificity towards nitrophenols in the presence of interferents. The stability of the sensor for nitrophenols was also investigated, and the fluorescence of the HP-SQDs/nitrophenols systems had no obvious change after the same solution was stored at 4 °C for 7 d (Fig. S7, ESI†), indicating that the sensor had good stability.

Sensing mechanism for nitrophenol isomers

The fluorescence quenching of HP-SQDs could occur by static quenching and PET between SQDs and nitrophenol isomers. The fluorescence decay curve of HP-SQDs, along with the addition of three nitrophenol isomers were recorded with the diode laser excitation at 297 nm. As shown in Fig. 6(a), the fluorescence lifetime from the single-exponential fitting of the decay curve was 4.06 ns for HP-SQDs. For HP-SQDs with the separate addition of o-NP, m-NP, and p-NP, the fluorescence lifetime was 3.90 ns, 3.90 ns, and 3.89 ns, respectively. After the addition of nitrophenols, the fluorescence lifetime of HP-SQDs exhibited only a slight change. For PET, the lifetime is shortened, F₀/F = τ₀/τ.^40,41

To confirm whether the PET is involved in the quenching mechanism, the E_red of HP-SQDs was determined from the electrochemistry as −0.71 V (Fig. S8a (ESI†), E_red is the onset potential of reduction for HP-SQDs from cyclic voltammetry), and the E_g was estimated as 4.75 eV (Fig. S8b (ESI†), E_g is the energy band gap derived from the absorption edge in the absorption spectrum of HP-SQDs). According to the empirical formula,^42,43 the E_HOMO and E_LUMO of the HP-SQDs were calculated as −8.44 and −3.69 eV, respectively (E_HOMO is the energy levels of the highest occupied molecular orbital and E_LUMO is the lowest unoccupied molecular orbital). The E_HOMO and E_LUMO of p-NP were calculated from the DMol3 DFT. As shown in Fig. S9 (ESI†), upon excitation, the electron in the HOMO of SQDs is excited to the LUMO and leaves the hole in the HOMO. As the energy of HOMO from p-NP lies between the LUMO and HOMO levels of SQDs, one electron from the HOMO of p-NP transfers to the HOMO of SQDs to fill the hole, which prevents the excited electron in the LUMO of SQDs from returning to the HOMO by the radiative pathway. The excited electron in the LUMO of SQDs then transfers to the HOMO of p-NP.^44,45 Therefore, the fluorescence of SQDs is quenched by the photoinduced electron transfer between HP-SQDs and p-NP.

The quenching data are presented as the plot of F₀/F versus [Q], along with the linear fitting, in Fig. 4(b), (d) and (f). The Stern–Volmer equation is given as eqn (3).⁴⁶

F0/F = 1 + Ksv[Q] = 1 + Kqτ0[Q]	(3)

where F₀ and F represent the fluorescence intensity of HP-SQDs and that of HP-SQDs with the addition of nitrophenol, respectively. K_sv is the Stern–Volmer quenching constant, and [Q] is the concentration of the quencher. K_q is the association constant for the complex formation. K_q for HP-SQDs and o-NP was about 2.06 × 10¹² and 1.43 × 10¹² for m-NP and HP-SQDs, respectively, and 4.80 × 10¹² M⁻¹ s⁻¹ for p-NP and HP-SQDs. These values all are much larger than the molecular diffusion rate constant (∼10¹⁰ M⁻¹ s⁻¹). The existence of static quenching by structural matching was proved, and the quenching constant of p-NP is significantly larger than that of the other two isomers.

Due to structural matching, the formation of the ground-state adduct will cause a perturbation of the frontier molecular orbitals, and then a change of the fluorophore's absorption spectrum. The absorption spectra of HP-SQDs with the addition of nitrophenols have significantly changed, indicating the structural matching results in the formation of non-fluorescent ground-state adducts of HP-SQDs and nitrophenol.

The significant changes in the absorption spectra, larger quenching rate and slight change of lifetime suggested that the fluorescence intensity decreased by structural matching and PET.

The quenching effect was more pronounced with the addition of p-NP than with o-NP and m-NP. This may be a result of the different structural matching degrees of nitrophenol. The less bulky p-NP may penetrate the cavity of cyclodextrin more easily than o-NP and m-NP. As displayed in Fig. 6(d), the spectral addition of HP-SQDs and p-NP is very different from that of HP-SQDs + p-NP. The UV-vis spectrum of p-NP has an intense absorption band around 320 nm, but it red-shifts to about 400 nm. This suggests that the inclusion inside the cavity of CD, and the binding has caused a chemical change probably by the deprotonation of phenol group. As shown in Fig. 6(b) and (c), the absorption spectrum of HP-SQDs + o-/m-NP is very close to the spectral addition of o-/m-NP and HP-SQDs, indicating that there is almost no strong interaction between o-/m-NP and HP-SQDs.

The structural matching between HP-β-CD and the three nitrophenol isomers was also investigated from the UV-vis spectra. As displayed in Fig. S10 (ESI†), nitrophenol isomers displayed an increased absorbance (hyperchromic effect) with the addition of HP-β-CD, suggesting that nitrophenol isomers have bound with the inner cavity of HP-β-CD. Furthermore, the inclusion constant (K) between HP-β-CD and the nitrophenol isomers is calculated as 2.95 × 10³ L mol⁻¹, K_m as 4.87 × 10³ L mol⁻¹ and K_p as 6.68 × 10³ L mol⁻¹ for o-, m- and p-NP, respectively, based on the Benesi–Hildebrand equation (Fig. S11, ESI†). The different inclusion ability of HP-β-CD with o-, m-, and p-NP may cause the different fluorescence quenching of HP-SQDs in the presence of nitrophenol isomers. The quenching data of HP-SQDs with the addition of nitrophenol isomers clearly indicated the strongest interaction with p-NP, consistent with the absorbance data for the inclusion with HP-β-CD.

Molecular modelling studies by quantum chemical calculations were conducted to gain more insightful details for the inclusion of nitrophenol isomers in HP-β-CD cavities. After optimizing each individual molecule, the structures corresponding to energy minima scans were further optimized at the M062X/6-31G* level. The binding model of nitrophenol isomers and HP-β-CD is presented in Fig. 7. The binding free energies of the nitrophenol isomers and HP-β-CD were estimated with Gaussian 16, and the value of each structure was corrected for the BSSE (eqn (2)), shown in Table S2 (ESI†). The binding free energy was −14.63 kcal mol⁻¹ for p-NP, which is lower than the binding energies for o-NP (−11.92 kcal mol⁻¹) and m-NP (−12.44 kcal mol⁻¹). The different substitution of the nitro group results in a different interaction of the nitrophenol isomers with HP-β-CD. The lowest energy of HP-β-CD/p-NP indicates the most stable complex. Thus, the most effective fluorescence quenching may be due to the highest structural matching between HP-SQDs and p-NP.

Method validation

The validation of nitrophenol detection by a HP-SQDs sensor was conducted by spiked water samples, including industrial wastewater, Jinyang Lake water, and tap water. As listed in Tables S3–S5 (ESI†), the standard recovery rate of p-NP is 101.7–103.9% (in industrial wastewater), 98.40–100.9% (Jinyang Lake water), and 95.67–101.3% (tap water), and the RSD are all less than 5.0% (n = 6). Also, the standard recovery rate of o-NP and m-NP are 98.10–102.3% and 98.29–102.8%, respectively.

The results show that this method can detect nitrophenol in real environmental samples. The reproducibility and specificity were checked by the cross-laboratory validation with matrix spikes. As shown in Table S6 (ESI†), the respective standard recovery rate of p-NP in matrix spikes is 101.6–101.9%. This indicates that the developed HP-SQDs sensor has great reproducibility and specificity towards the detection of p-NP.

Conclusions

HP-SQDs were prepared by the oxygen accelerated activation method using sublimated sulphur powder and HP-β-CD as a surface modifier. The synthesized HP-SQDs have the great advantages of good water solubility, high photostability and fluorescence, as well as molecular cages. Three nitrophenol isomers have different structural matching within the cages of HP-SQDs. The lowest binding energy indicated the formation of the most stable ground state complex, which resulted in the most effective fluorescence quenching of HP-SQDs by p-NP. Therefore, HP-SQDs have the best detection result for p-NP. The sensing linear range was from 2.5 to 45 μmol L⁻¹ for p-NP, and the detection limit was 0.25 μmol L⁻¹ (3σ/k). HP-SQDs were also applied for the detection of nitrophenols-spiked water samples, with a good recovery rate. This study proposed a new fluorescent cage sensing based on structural matching, which may be a great approach for the detection of isomers.

Author contributions

Yining Chang: data analysis, writing the original draft. Ran He: Original research proposal, data curation and analysis. Runqiu Wang: data analysis. Yanli Wei: experimental supervision and rationalization of research proposal, verification of data analysis and proofreading. Li Wang: project administration, supervision, verification of data analysis, editing and proofreading.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by Shanxi Provincial Hundreds of Talents Programs with the grant no. 205698901008, 205698901012 and 205698901004, Shanxi Special project for patent promotion of Shanxi Province (202202034) and the earmarked fund for Modern Agroindustry Technology Research System (2024CYJSTX10). The authors thank Ms Yang Yang (PhD student at the Institute of Molecular Science, Shanxi University) for the theoretical calculation.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc02501f

This journal is © The Royal Society of Chemistry 2024