A reversible redox-fluorescence switch based on Prussian blue and carbon quantum dots for dual-spectral detection of N₂H₄ and H₂O₂

Jiayu Gao ^a, Yuan Zhang ^b, Ying Sun ^a, Siyue Wang ^a, Zhelin Liu *^a, Bo Zhao *^a, Xiangting Dong ^a and Shouhua Feng ^b
aSchool of Chemistry and Environmental Engineering, Changchun University of Science and Technology, Changchun, Jilin 130022, P. R. China. E-mail: zhelinliu@cust.edu.cn; bozhao@cust.edu.cn
^bState Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry, Jilin University, Changchun, Jilin 130012, P. R. China

First published on 24th August 2024

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Introduction

Fluorescence detection plays a crucial role in optical detection as it offers high sensitivity and easy operation, which makes it applicable in various fields including medicine, food, and environment.^1–3 By continuously switching between fluorescence quenching and recovery states, the concept of a fluorescence switch has been proposed. Li et al.⁴ reported a ternary “ON–OFF–ON” fluorescent switch based on graphene quantum dots. They proposed a new cation control strategy, which provided a new prospect for the research of “ON–OFF–ON” fluorescent switches in the future. In the past decade, fluorescent switches have been applied in metal ion detection, pesticide detection, data security and other fields.^5–8

In the fabrication of a fluorescent switching system, the selection of a quencher and a fluorophore is especially critical. Prussian blue (PB), also referred to as ferric ferrocyanide, was initially developed as a dye and its electrochemical properties were reported until 1978. Since then, it has been widely used in biosensors, ion batteries, electrocatalysis, and nanozymes due to its excellent electrochromic properties, good biocompatibility, prominent photothermal effect, biosafety, and peroxidase-like activity.^9–12 In addition, PB is a drug approved by the food and drug administration (FDA) for the treatment of thallium poisoning.¹³ Furthermore, PB is also utilized as an optical sensor owing to its excellent reversibility between oxidation/reduction states and the presence of a characteristic electronic transition band in the range of 550–1000 nm. Yu et al.¹⁴ proposed a colorimetric biosensor using a green substance which is composed of PB and potassium ferricyanide as an indicator for the detection of toxic substances in an aqueous environment. The color of PB is dark blue, and when it is reduced to Prussian white (PW), it becomes colorless and the UV-vis absorption peak decreases. Therefore, PB can undergo transitions between oxidized and reduced states under different conditions.

Carbon quantum dots (CDs) are a new class of zero-dimensional nanomaterials, which can be better dispersed in water and have unique properties such as good biocompatibility and low toxicity compared to traditional lead-based and cadmium-based quantum dots.^15,16 Since their initial reporting in 2006, carbon dots have been widely employed in diverse areas, including sensing, catalysis, and bioimaging.^17–19 Eksin et al.²⁰ developed a carbon quantum dot-modified disposable pencil graphite electrode for the first time, and utilized it for sensing drug–DNA interactions. CDs have been well applied to the construction of various fluorescent nanosensors in the past decade due to their stable photoluminescence properties.²¹ Jiang et al.²² developed a fluorescent colorimetric paper strip based on carbon dots/gold nanoclusters, enabling the visual and quantitative detection of iodine ions in urine for health assessment. This strategy can be applied to home medical testing and timely health screening.

Hydrazine hydrate (N₂H₄) is an important chemical raw material due to its strong nucleophilic and reducing properties, and its detection is of significant importance. There are many methods to detect hydrazine hydrate, including electrochemical and chemical titration, spectrophotometry, high-performance liquid chromatography, etc.^23–25 But most of these methods suffered from the shortcomings such as complicated instrumental operation, long time-consumption and the difficulty of sample preparation, which limited their practical applications. Meanwhile, hydrogen peroxide (H₂O₂) exhibits strong oxidizing and bleaching properties, encompassing a vast array of applications in industries such as paper, food, and metal smelting. Due to its colorless and odorless properties, as well as its ease of preparation, hydrogen peroxide is commonly used as a food additive, a bleaching agent, an antiseptic, and a deodoriser.²⁶ Therefore, the detection of hydrogen peroxide is very important. The detection methods for hydrogen peroxide mainly include capillary chromatography, electrochemical methods and high-performance liquid chromatography.^27–29 Although the above methods can achieve the purpose of H₂O₂ detection, they are often limited by a long sample preparation time, expensive large-scale instruments and complicated operation. Compared with the above-mentioned traditional detection methods, fluorescence switches can achieve the desired detection performance with simple operation, low cost, a wide detection range and a low detection limit towards small molecule detection. To the best of our knowledge, the combination of PB and CDs has never been used for dual-spectral detection of N₂H₄ and H₂O₂.

In this work, we successfully prepared PB@CD/SiO₂ nanoparticles by using PB as a quencher and CDs as a fluorophore. CDs were encapsulated in a silica layer on the surface of PB nanoparticles (NPs) through the hydrolysis of tetraethyl orthosilicate. PB can transform between the reduced and oxidized states by adding hydrazine hydrate and hydrogen peroxide, resulting in the change between “ON” and “OFF” states of the fluorescence switch.

Experimental section

Chemicals

Potassium ferricyanide, polyvinylpyrrolidone (PVP), hydrochloric acid, p-phenylenediamine (p-PDA), nickel chloride, dichloromethane, isopropanol (IPA), ammonia, and tetraethyl orthosilicate (TEOS) were used. All of these reagents were analytically pure. Water used in the experiments was ultrapure water with resistivity ≥18.2 MΩ cm.

Preparation of PB NPs

PB NPs were prepared according to the previously reported literature with a slight modification.³⁰ After dissolving 3.8 g PVP in 80 mL water under stirring at room temperature, 0.28 g potassium ferrocyanide was added to the solution. Subsequently, 0.8 mL hydrochloric acid was added, and the resulting solution was transferred to a 250 mL round-bottomed flask and stirred at 80 °C for 12 h to produce a deep blue solution. The prepared product was washed three times with water and ethanol, then subjected to vacuum drying at 60 °C overnight to obtain PB NP powder.

Preparation of CDs

The preparation of CDs was carried out according to the previous report with a slight modification.³¹ Upon the addition of 0.05 g p-PDA and 0.137 g NiCl₂·6H₂O to 50 mL water, a mixture was formed that was transferred into a 100 mL Teflon liner. Subsequently, the reaction was conducted at 160 °C for 6 h. As the reaction ended, the resultant mixture was subjected to centrifugation at 9500 rpm for 20 minutes and the supernatant was collected. The obtained supernatant was extracted by dichloromethane to yield a pale-yellow oily liquid, and subsequently concentrated by rotary evaporation. The resulting product was dispersed in ethanol for future use.

Preparation of PB@CD/SiO₂ NPs

In a typical preparation, 20 mg PB was dispersed in 15 mL IPA, followed by the addition of 0.5 mL of CD dispersion to the solution. The mixture was stirred for 5 minutes. Subsequently, 1 mL NH₃·H₂O and 200 μL TEOS were dropped into the solution and stirred at room temperature for 12 h. The resulting solution was washed three times with ethanol and ultrapure water, and vacuum dried at 60 °C for 3 h to attain PB@CD/SiO₂ NP powder.

Characterization

Scanning electron microscopy (SEM, JEOL JSM7610F) and transmission electron microscopy (TEM, Hitachi H-600) were used to characterize the morphology of the products. X-Ray diffraction (XRD) measurements were performed on a D/max 2550V/PC X-ray diffractometer using Cu Kα radiation (40 mA, 40 kV) at a scan rate of 8° min⁻¹. Fourier transform infrared (FTIR) spectra were collected on a FTIR-8400s FTIR spectrometer. Energy-dispersive X-ray energy (EDX) mapping images were obtained on a FEI TECNAI F20 TEM equipped with an EDX detector. Photoluminescence (PL) studies were carried out on a F97Pro fluorescence spectrophotometer, while UV-vis absorption spectra were obtained on a 759S UV-visible-near infrared (NIR) spectrometer.

Results and discussion

Characterization of PB@CD/SiO₂ NPs

In this study, PB@CD/SiO₂ NPs were prepared by a simple and effective method. SEM was used to study the morphologies of the prepared PB and PB@CD/SiO₂ NPs. The SEM image of PB is depicted in Fig. 1(a), showing a cubelike structure with uniform particles. Fig. 1(b) displays the SEM image of the PB@CD/SiO₂ NPs, in which a rougher surface of PB@CD/SiO₂ NPs than that of PB can be seen. The structures of the prepared materials were further analyzed by TEM. Fig. 1(c) and (d) presents the TEM images of PB and PB@CD/SiO₂ NPs. It is evident that PB@CD/SiO₂ exhibits a core–shell structure with a cubic shape. In comparison with the prepared PB, it can also be seen that an outer layer is wrapped around the core on PB@CD/SiO₂, suggesting that the SiO₂ layer has been coated on the PB surface successfully. Besides, the slight change in the morphology and particle shape may be due to the partial degradation of PB in alkaline media during the composite preparation.

The elemental composition of PB@CD/SiO₂ NPs was analyzed by EDX mapping analysis. Fig. 2 displays the TEM (a) and corresponding EDX elemental mapping (b)–(g) images of PB@CD/SiO₂ NPs. It can be seen that Fe, N and K are concentrated in the core region of the composite, while Si and O exist in the outer shell region, showing the core–shell structure. Besides, C exists in both the core and shell regions, indicating its existence in the core region as PB and shell region as CDs. EDX mapping results imply a successful combination of PB and the CD-containing silica layer.

Fig. 3 illustrates the FTIR spectra of PB (a), SiO₂ (b), PB@CD/SiO₂ (c) and CD (d). Fig. 3(a) presents a notable absorption peak at 2098 cm⁻¹, which is assigned to the CN group in PB.³² Additionally, the peak observed at 1632 cm⁻¹ is attributed to CO in CDs, and the peaks located at 1260 cm⁻¹ and 1130 cm⁻¹ are associated with the C–N stretching vibration and C–O stretching vibration, respectively, as shown in Fig. 3(d).³¹ The peaks at 467 cm⁻¹ and 1090 cm⁻¹ in Fig. 3(b) are attributed to the O–Si–O bending vibration and asymmetric stretching vibration in SiO₂.³³ Moreover, the band features of PB, SiO₂ and CD can be seen on the PB@CD/SiO₂ nanocomposite (Fig. 3(c)), further indicating the presence of PB, SiO₂ and CD in the composite. According to the above results, PB and the silica layer containing CDs were successfully bonded.

To further investigate the chemical composition of the composite, X-ray diffraction was also performed. Fig. 4 shows the X-ray diffractogram of PB and PB@CD/SiO₂ NPs, and the positions of the diffraction peaks are consistent with pure Prussian blue PDF#73-0687, indicating the successful preparation of PB and the existence of PB in the composite. Besides, the combination of PB and the CD-containing SiO₂ layer does not affect the crystal structure of PB.

The formation scheme of the PB@CD/SiO₂ composite is shown in Fig. 5(a). In this work, PB and CD were firstly prepared, then the silica layer containing CDs was coated on the surface of PB NPs through the hydrolysis of TEOS under alkaline conditions. The prepared PB@CD/SiO₂ composite fluorescent switch is further used for the detection of N₂H₄ and H₂O₂, and the scheme is shown in Fig. 5(c). PB has a strong and broad absorption peak within the wavelength range of 550 to 1000 nm in the UV-vis absorption spectrum, whereas CDs show an emission peak in the fluorescence spectrum, as shown in Fig. 5(b). However, based on the internal filtering effect (IFE), PB can act as an absorber and absorbs the fluorescence emitted by CDs. Moreover, the fluorescence of the composite can be quenched, and the emission peak at 652 nm disappears. The cross-linked network structure of the silica shells formed by TEOS hydrolysis allows small molecules to pass through. Therefore, upon the addition of N₂H₄, PB is reduced and converted to PW, while the absorption intensity of the composite in the 550–1000 nm wavelength range is significantly reduced. It should be noted that the addition of N₂H₄ has no effect on CD. Therefore, when PB is converted to PW, the emission peak at 652 nm appears at an excitation wavelength of 470 nm, and the color of the dispersion changes from dark blue to nearly transparent (Fig. 5(b) inset), showing an “ON” state (Fig. 5(c)). When H₂O₂ is added, PW is converted to PB, and the PL response of CDs is covered by the increased UV absorption of PB. The presence of IFE causes the fluorescence of the composite to return to a quenched state, showing an “OFF” state. As a result, the composite does not show a significant emission peak at 652 nm under 470 nm excitation, and the color of the dispersion changes again to dark blue. With the addition of N₂H₄ and H₂O₂, PB continuously transforms between the reduced and oxidized states, and the UV and PL responses simultaneously change accordingly.

Spectral detection of N₂H₄ on the PB@CD/SiO₂ composite

Hydrazine hydrate is an important raw material in industrial production, and can cause damage to the human respiratory system, the liver system and the nervous system through oral and nasal respiration and skin penetration.^34,35 In this study, N₂H₄ was detected by both UV-vis absorption and fluorescence spectroscopies. The response time of the composite to N₂H₄ was firstly investigated before detection. As 100 μM N₂H₄ was added to the composite, its fluorescence intensity enhanced. It was found that the absorbance of the composite no longer increased after 540 s, indicating a response time of 540 s to N₂H₄. As mentioned above, with the addition of hydrazine hydrate, PB is converted to PW, leading to a decrease in UV-vis absorption and an increase in fluorescence intensity. The UV-vis absorption spectral response of PB@CD/SiO₂ to hydrazine hydrate was firstly investigated. As shown in Fig. 6(a), the absorption intensity of the PB@CD/SiO₂ composite decreased with the increase of the N₂H₄ concentration. This decrease was attributed to the reduction of PB to PW in the composite as the N₂H₄ concentration increased, resulting in the disappearance of the electronic transition band between 550–1000 nm. Fig. 6(b) presents the absorption fitting curves of the PB@CD/SiO₂ composite at various N₂H₄ concentrations. The absorbance of the composite showed linear correlation with the N₂H₄ concentration in the range of 0–100 μM (R² = 0.9970) and 100–1100 μM (R² = 0.9957). According to the formula C_L = kS_N/m (where C_L represents the detection limit, k is the confidence factor, S_N is the standard deviation of the blank test, and m is the slope of the standard curve), the detection limit was calculated to be 0.11 μM.

The fluorescence response of PB@CD/SiO₂ was subsequently tested. Fig. 6(c) shows the N₂H₄ fluorescence response of the composite, while Fig. 6(d) depicts the fitting curve of the fluorescence intensity as a function of the concentration of N₂H₄ when different concentrations of N₂H₄ were added to PB@CD/SiO₂. The results indicate linear correlation between the N₂H₄ concentration and fluorescence intensity in the composite when concentrations of hydrazine hydrate were 0–100 μM (R² = 0.9921) and 100–1100 μM (R² = 0.9974). The detection limit was calculated to be 0.15 μM. Compared with other detection systems in Table 1, the detection range of the prepared composite is wider and the detection limit is lower.

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Spectral detection of H₂O₂ on the PB@CD/SiO₂ composite

In this study, bispectral detection of H₂O₂ was performed through changes in UV absorption and fluorescence intensities of the PB@CD/SiO₂ composite with the addition of H₂O₂ at diverse concentrations. The response time of the composite to H₂O₂ was examined before the detection. As 100 μM H₂O₂ was added to the composite, its fluorescence intensity decreased. It was observed that the absorbance of the composite ceased to decrease beyond 360 s, indicating a response time of 360 s for the composite to detect H₂O₂. In Fig. 7(a), the absorption spectra show the change in absorption intensity as a function of H₂O₂ concentration in the presence of 1100 μM N₂H₄. The concentration of hydrogen peroxide added to the composite varies from 0–800 μM. It can be observed that the absorbance of the composite between 550–1000 nm increases with the increase in the H₂O₂ concentration. Fig. 7(b) illustrates the absorption plot fitted with various H₂O₂ concentrations. An obvious linear relationship can be seen when the concentration of H₂O₂ is 0–200 μM (R² = 0.9885) and 200–800 μM (R² = 0.9915). The limit of detection was calculated to be 0.93 μM.

Fig. 7(c) explores the PL response of the composite to hydrogen peroxide. As the absorption intensity of the composite increases between 500–1000 nm, the red light emitted by CDs in the composite is completely absorbed. Consequently, the fluorescence switching system is in an “OFF” state, resulting in a gradual decrease in fluorescence intensity of the composite. Fig. 7(d) shows the fitted curve of the fluorescence intensity of PB@CD/SiO₂ with various H₂O₂ concentrations. The fluorescence intensity of the composite shows a linear correlation with the H₂O₂ concentration within the range of 0–560 μM (R² = 0.9898) and 560–960 μM (R² = 0.9945). The calculated limit of detection was 1.41 μM. As shown in Table 2, it can be seen that PB@CD/SiO₂ has a good performance for H₂O₂ detection with a lower detection limit and a wider detection range.

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The reversibility of fluorescence switching also needs to be taken into account for practical applications, so we monitored the spectral response of PB@CD/SiO₂ in the oxidized/reduced state for 10 cycles of switching. As shown in Fig. 8(a), it can be seen that after ten cycles, the fluorescence can still be restored with 80% of the original intensity. Similarly, the UV-vis absorption intensities also demonstrate a strong trend of reversibility, as they still maintain over 97% of the initial state even after ten cycles (Fig. 8(b)). This suggests that the prepared composite has excellent reversibility and fatigue resistance.

Conclusions

In summary, we constructed a PB@CD/SiO₂ fluorescence switch by coating a CD-containing SiO₂ layer on the PB surface. The morphology and chemical composition of the composite were analyzed by means of SEM, TEM, EDX, XRD, and FTIR analyses. As is known, PB has a broad and intense absorption peak within the wavelength range of 550–1000 nm. According to the inner filter effect, the emission peak produced by CD photoluminescence at 652 nm disappeared, leading to fluorescence quenching and presenting an “OFF” state. In the presence of N₂H₄, PB can be transformed into PW, leading to the recovery of fluorescence. Subsequent introduction of H₂O₂ induces the reversion of PW to PB, resulting in the fluorescence quenching. This dynamic process enables the composite fluorescence state to toggle between “ON” and “OFF”. As a result, PB@CD/SiO₂ can be employed as a platform to detect N₂H₄ and H₂O₂ with a broad detection range and a fast response. The fluorescence intensity at 652 nm exhibits a linear correlation with the N₂H₄ concentration, showing a detection limit of 0.15 μM. Additionally, the UV-vis absorption intensity at 750 nm shows a linear dependence on the H₂O₂ concentration, with a detection limit of 0.93 μM. This work provides a simple and effective method for the detection of N₂H₄ and H₂O₂, which has good application prospects in environmental analysis.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Scientific and Technological Project of Jilin Province (grant numbers 20210101136JC, 20200201233JC).

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