Introduction

Sulfamethazine (4-amino-N-(4, 6-dimethyl-2-pyrimidinyl) benzene sulfonamide, SMZ), as one of the common antibiotics, was utilized in human and veterinary medicine [1,2,3]. It is reported that SMZ is widely presented in groundwater, surface water, and soil sediments [4]. The content of SMZ in the environment is generally between nanograms per liter and micrograms per liter. Although EU countries have stopped using sulfamethazine in agriculture since 2006 [5], however, sulfonamide antibiotics were still widely used in feed additives to improve livestock and poultry growth [6]. Overdosing or chronic abuse of SMZ may cause the accumulation in the human body [7]. EU, USA, and China have established the maximum residue limit of SMZ as 89.8 nM in milk [8]. In order to measure SMZ sensitively and quantitatively, the development of an efficient and simple detection system is still necessary.

At present, the fluorescence/chirality [9], enzyme-linked immunosorbent assay (ELISA) [10], electrochemical immunosensor [11], and ultra-high-performance liquid chromatography spectrometry-mass spectrometry (UHPLC-MS/MS) [12] have been developed for detection of SMZ. Consequently, it is necessary to develop a high-selectivity and low-cost-effective sensors for the detection of SMZ. In recent years, gold nanoparticles (AuNPs) have been considered for applications in different fields such as medicinal coverings [13], catalysis [14], and electrode materials [15] due to their good significant optical, high electrical conductivity, suitable redox property, and stability [16]. Among them, AuNP-based colorimetric analysis has been developed widely due to its rapidity and convenience. The macrocyclic supramolecules, such as cyclodextrin (CD), crown ether, and calixarene, possess unique and size-tunable cavity structures and exhibit special recognition properties [17]. Due to their great chemical stability, facile synthesis, and easy functionalization, the hybrid nanomaterial and macrocyclic supramolecules have been extensively studied [18,19,20,21,22,23,24,25,26]. Para-sulfonated calix[n]arene (pSCn) has open, rigid cavities and is water-soluble, which was widely used as recognition molecule.

In this work, we designed a label-free colorimetric sensor platform based on pSC4-AuNPs. Para-sulfonatocalix[4]arene (pSC4) capped gold nanoparticles (pSC4-AuNPs) were synthesized using one-pot methods [27]. The pSC4 has been confirmed to bind with the -NH2 group selectively, which has the ability to recognize the amine group containing molecules. SMZ contains -NH2 and sulfonamide groups at both ends of molecule, which is possible to bind two pSC4 through host–guest interaction [28]. Here, pSC4 was used as SMZ’s potential recognition host for developing simple, ultrasensitive, and rapid colorimetric detection approaches towards SMZ. Due to the specific binding between SMZ and pSC4, gold nanoparticles aggregate in the presence of SMZ and result in the color changes. As shown in Scheme 1, the SMZ binds to pSC4-AuNPs via host–guest recognition between the -NH2 and sulfonamide groups of SMZ and the macrocyclic cavity of pSC4. With the existence of target molecule, the aggregation of pSC4-AuNPs will be observed. Therefore, combining the properties of gold nanostructures and supramolecular chemistry, the fabricated colorimetric sensor has good specificity and sensitivity.

Scheme 1
scheme 1

Schematic illustration of ultrasensitive UV visualization of SMZ based on host–guest recognition

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Experimental

Chemicals and materials

Para-sulfonatocalix [4] arene (pSC4) was obtained from TCI Development Co., Ltd. (Shanghai, China). Sodium borohydride (NaBH4), potassium chloride (KCl), disodium hydrogen orthophosphate (Na2HPO4), and hydrogen tetrachloroaurate trihydrate (HAuCl4⋅3H2O) were obtained from Sigma-Aldrich, Inc. (St. Louis, USA). Sulfamethazine (SMZ), tetracycline (TC), enrofloxacin (ENR), norfloxacin (NOR), oxytetracycline (OTC), sulfamoxole (SMX), and levofloxacin (LEV) were purchased from Sangon Biotech Co., Ltd. (Shanghai, China). All the aqueous solutions were prepared with ultrapure water (18.2 MΩ cm) from a Millipore Milli-Q purification system (Barnstead, USA).

Apparatus

The UV–vis spectra were carried out with a Shimadzu UV-2450 spectrophotometer (Japan), using a quartz cuvette having a volume of 50 μL and a path length of 10 mm. Transmission electron microscopy (TEM) image was obtained with the JEM-2010F instrument (Japan). Fourier transform infrared (FT-IR) spectra were recorded by using a spectrum FTIR 6300 spectroscopy system (JASCO, Japan). Dynamic light scattering (DLS) and zeta-potential experiments were obtained by a Malvern Zetasizer 3000 HS system (Malvern Instruments, UK). SPR measurements were performed by the MP-SPR Navi 210A VASA instrument (BioNavis Ltd, Tampere, Finland).

Preparation of the pSC4-AuNPs and recognition of SMZ by pSC4-AuNPs

Typically, the pSC4-AuNPs were prepared according to previous research [29]. SMZ is almost insoluble in water and easily soluble in dilute hydrochloric acid, sodium hydroxide, or an ammonia solution. To make the stock solution, a quantity of 2.85 mg of SMZ was dissolved into 1 mL of 0.25 M diluted sulfuric acid solution and stored at 4 °C for subsequent experiments. Subsequently, pSC4-AuNPs (90 μL) were prepared by dropping 10 μL of SMZ into a glass beaker at room temperature. The solution was continuously mixed with a vortex mixer for 2 min and measured with UV–vis spectra.

Selectivity detection of SMZ

To demonstrate that the proposed detection method had good selectivity of SMZ, the ENR, NOR, LEV, SMX OTC, TC, and SMZ as known to be the present substances in environmental water were added into the reaction system. Various antibiotics solutions (10 μL, 10 nM) were mixed with 90 μL nanoparticle solution in range of 2.5 ~ 80 nM. All determines were recorded by the UV absorption spectrum at 520 nm and 675 nm. The equilibrium dissociation constant of pSC4-AuNPs to SMZ was determined using an SPR binding experiment. Different concentrations of SMZ were flowed and reacted on the pSC4-AuNPs modified gold chip for about 40 min for the detection process (1 nM, 10 nM, 100 nM, 1 μM, 10 μM, 100 μM).

Real-sample detection of SMZ

To validate the practicability of this established method, the sensing of SMZ was accomplished in real samples. The determination of SMZ in deionized, tap, and lake water were investigated using the standard addition method. The tap water and lake water were filtered through a 0.22 μm PTFE syringe filter before sensing, samples (deionized, tap, and lake water) do not contain sulfamethazine. The 20 nM concentration of SMZ was added to deionized, tap and river water solutions which added 80 μL of pSC4-AuNPs and 20 μL SMZ mixed buffer. Therefore, the final concentration of sulfamethazine in the samples is 20 nM after spiking. The 675 nm UV–vis absorbance value was used for further actual sample analysis. All of the experiments mentioned above were repeated three times. Under optimal experimental conditions, the determination of SMZ in deionized, tap and river water was investigated using the standard addition method.

Results and discussion

Characterization of the synthesized pSC4-AuNPs and pSC4-AuNPs/SMZ aggregates

The pSC4-AuNPs were prepared via a simple and efficient one-step synthesis procedure. To prove the principle of the detection system, the host–guest recognition and electrostatic interaction between SMZ and pSC4-AuNPs were measured by UV–vis spectroscopy. As shown in Fig. 1A, the UV–vis absorption spectrum of pSC4-AuNPs was observed at 520 nm. After adding the SMZ, the peak at 520 nm was decreased and a new peak at 675 nm appeared. These results indicate that the SMZ can successfully make the aggregation of gold nanoparticles via host–guest recognition between the -NH2 and sulfonamide groups of SMZ and the macrocyclic cavity of pSC4 [30]. In the Flourier transform infrared spectroscopy (FT-IR) spectra of pure pSC4 and AuNPs modified with pSC4 (Fig. 1B), the stretching vibration peaks of -SO3− were found at 1195 and 1049 cm−1, which shifted to 1164 and 998 cm−1 after being modified to AuNPs, indicating that the -SO3− groups coordinate with the gold atoms on the surface interaction of the AuNPs [31]. To further study the SMZ controlled aggregation of pSC4-AuNPs, the TEM was used to characterize pSC4-AuNPs samples before and after the addition of SMZ. As shown in Fig. 1C, uniform gold nanoparticles with a diameter of 14 nm are observed. TEM images of Fig. 1D showed the aggregation structure of pSC4-AuNPs clearly and the inset of Fig. 1D shows the higher magnifier. These results indicate that the SMZ can successfully regulate the aggregation of gold nanoparticles. Figure 1E shows that the zeta-potential of pSC4-AuNPs changes from − 26.8 to − 19.4 mV after interaction with SMZ. Also shown in Fig. 1F, the particle diameter of pSC4-AuNPs/SMZ was 399.8 nm, which is larger than that of pSC4-AuNPs (39.6 nm). Above of all, the pSC4 on the surface of AuNPs can recognize SMZ that leads to the obvious aggregation of the pSC4-AuNPs.

Fig. 1
figure 1

A UV–vis spectra of the pSC4-AuNPs (red trace,), pSC4-AuNPs/SMZ (black trace), and pSC4-AuNPs/H2SO4 (blue trace). B FT-IR spectra of pSC4 and the pSC4-AuNPs. TEM images of pSC4-AuNPs (C). TEM images of pSC4-AuNPs aggregation (D). Zeta-potential profile spectra (E) and size of pSC4-AuNPs(F) before and after in the presence of SMZ

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The feasibility and condition optimization

The effects of the reaction time of SMZ and the pSC4-AuNPs were investigated using UV–vis absorption spectroscopy. The absorption peak of UV–vis at 520 nm represents the dispersed pSC4-AuNPs, while the absorbance value at 675 nm showed the aggregation peak [32]. Therefore, the conjugation of pSC4-AuNPs and SMZ was measured based on UV–vis absorption spectra (Fig. S2 A). The absorbance ratio of A675/A520 was used to quantify SMZ-induced aggregation (Fig. S2 B). The results showed that the host–guest reaction of pSC4 and SMZ on the surface of AuNPs resulted in a gradual aggregation process. The A675/A520 value rises rapidly at the first stage of 10 min and then remains almost constant, indicating that SMZ and pSC4-AuNPs have completely reacted after 10 min. Therefore, the reaction time of 10 min was chosen as the reaction conditions for the subsequent study.

Performance of the constructed colorimetric sensor

Colorimetric investigation for detection of SMZ was carried out under the room temperature. As shown in Fig. 2A, with the concentration of SMZ increases, the gradual aggregation of nanocomposites causes the color of the pSC4-AuNPs solution to gradually change from red to gray. Figure 2B reveals linear relation between SMZ concentrations and UV–vis signals. The values following a linear behavior were observed for concentration range of 2.5 ~ 20 nM by the regression equation y = 1.5 × 10–2 [SMZ] + 8.7 × 10–2 with r2 = 0.9908 (inset of Fig. 2B). The limit of detection (LOD) can be calculated from the equation LOD = 3σ/s, where σ is the standard deviation of blank signal and s is the slope of regression equation. The LOD was found to be 1.39 nM. The performance of SMZ colorimetric sensors was compared with the reported sensors (Table 1). Compared with those sensors, such as LC–MS, enzyme-linked aptamer or fluorescence methods, the fabricated method has a narrow detection window and higher LOD. However, the synthesis of pSC4-AuNPs was very simple, which avoids expensive antibody costs. Also, the detection limit reached the national maximum residue limit and the detection process was also very fast. Therefore, this colorimetric on-site detection method is still competitive in terms of rapid detection and simple fabrication.

Fig. 2
figure 2

A Visible spectra with the different concentrations of SMZ. The color change on the bottom from left to right represents SMZ at the concentrations of 0, 2.5, 5.0, 10, 15, 20, 40, 60, 80 nM, respectively (inset: pSC4-AuNPs solution before (left) and after (right) adding of SMZ with different concentrations). B Calibration curve representing absorption A675 versus concentration of SMZ in 10 nM (Inset: linear range with 2.5, 5, 10, 15, 20 nM for SMZ detection)

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Table 1 Comparison of LOD value for SMZ detection performance of pSC4-AuNPs with other methods
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Specificity detection and stability of the pSC4-AuNPs

To assess the sensor selectivity, the same concentrations of tetracycline (TC), enrofloxacin (ENR), norfloxacin (NOR), oxytetracycline (OTC), sulfamoxole (SMX), and levofloxacin (LEV) were added in the colorimetric sensor. From Fig. 3A, we can observe that the binding of pSC4-AuNPs to SMZ is significantly different from others. In the ultraviolet spectrum of the pSC4-AuNPs system, there is no obvious change when introducing other antibiotics except the SMZ. In addition, the result of Fig. S1 also showed that the equilibrium dissociation constant of SMZ and pSC4-AuNPs was 98.9 nM. It is indicated that pSC4-AuNPs has higher affinity toward sulfamethazine. With the addition of SMZ, the ratio of the absorbance intensity at A520 nm and A675 nm (A675/A520) was proportionally increased, suggesting an excellent selectivity to SMZ. The results showed that all of the presented coexisting substances have no interference in the detection of SMZ for the reported pSC4-AuNPs system, which demonstrates the constructed colorimetric sensor has a good selectivity and specificity. The stability of the SMZ sensor was evaluated by UV–vis absorbance of pSC4-AuNPs for 14 weeks. As shown in Fig. 3B, the absorbance of pSC4-AuNPs did not significantly change after 14 weeks of storage, which indicated the good stability of pSC4-AuNPs.

Fig. 3
figure 3

A The variations in the absorbance ratio A675/A520 colorimetric pSC4-AuNPs detection system in the presence of ENR, NOR, LEV, SMX OTC, TC, SMZ. Blank means pure water only. B Stability of pSC4-AuNPs for long term storage

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Detection in tap and lake water samples

The feasibility and reliability of colorimetric sensor for the determination of SMZ in deionized, tap, and river water was investigated using the standard addition method. The 20 nM concentrations of SMZ were added into real samples. Each analysis was conducted in triplicate to reduce the experimental error. As shown in Table 2, the high recoveries (90.18–107.06%) were obtained, and RSD ranged from 1.21 to 2.05%. For river or lakes samples, the results indicated that this SMZ sensor can be applied in real samples determination even though the water environment is complicated.

Table 2 Analysis results of water samples spiked with SMZ by the proposed sensor (n = 3)
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Conclusions

In this work, a sensitive and facile method was developed based on the host–guest interaction for aqueous determination of SMZ. SMZ mediates the aggregation of pSC4-AuNPs through the host–guest recognition, which can be used as a simple and on-site colorimetric SMZ assay. Compared with other sensors, it has a small disadvantage in terms of versatility, which has low specificity and a narrow detection window. Take into account of the simple synthesis of pSC4-AuNPs and fast detection of SMZ, the proposed strategy paves the way for the development of a simple and sensitive platform for the rapid detection of interactions between small molecules and macrocycles.