A novel Fe₃O₄/CdTe fluorescence probe for sialic acid detection based on a phenylboronic acid–sialic acid recognition system

Abstract

A novel fluorescence method was established for detecting sialic acid (SA) by using phenylboronic acid (PBA)-functionalized Fe₃O₄/CdTe magnetic nanoparticles as fluorescence probe. Initially, Fe₃O₄ nanoparticles were modified with amino groups, and then 3-mercapto propionic acid-stabilized CdTe quantum dots (QDs) were covalently linked to the amino-modified Fe₃O₄ nanoparticles to form Fe₃O₄/CdTe magnetic fluorescence nanoparticles. Finally, PBA was introduced on the surface of Fe₃O₄/CdTe to form PBA-functionalized Fe₃O₄/CdTe magnetic fluorescence nanoparticles. This kind of nanoparticle can specifically recognize SA and its fluorescence intensity is quenched by SA. In addition, the conjugate of the nanoparticles and SA is easy to separate from the sample matrix due to its magnetic properties. Therefore, this nanoparticle can be used as a fluorescence probe to detect SA. Under optimal conditions, the fluorescence intensity of the nano probe was found to be inversely linear with the concentration of SA in a wide range of 50 μg ml⁻¹–1.50 mg ml⁻¹, and the limit of detection was 16 μg ml⁻¹. The PBA-functionalized Fe₃O₄/CdTe nano probe was applied to the determination of SA in infant formula, and the result showed high accuracy and precision. The PBA-functionalized Fe₃O₄/CdTe nano probe has the potential to be used to monitor SA in food.

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1. Introduction

Sialic acid (SA) is a kind of 9-carbon monosaccharide derivative, frequently found at the terminal position of glycoconjugates.¹ There are animal models confirming that insufficiency of SA intake influences mental development and learning ability.² Since SA plays an important role in brain growth, some famous food companies have recently launched the “SA plan”,³ which is to increase the content of SA in infant formula. However, excessive SA intake leads to the occurrence of inflammation in the body.⁴ So, it is very important to monitor the quantity of SA added into infant formula. Some methods including colorimetry, chromatography, fluorescence and enzymatic assays are powerful for SA detection,^5–8 but these techniques have their own defects. Chromatography has high sensitivity and specificity, but it needs expensive instruments and professional operation. Although enzymatic assay of SA has shown great promise, it is limited by either expensive enzymatic reagents or being time-consuming. Substance interference in complex samples is the limitation of the colorimetry and fluorescence assays. So, there is a practical need to develop new approaches for determination of SA to overcome the shortcomings of the present strategies.

Recently, quantum dots (QDs) as fluorescence probes have attracted increasing interest in determination of saccharides due to their unique optic characteristics.^9–11 Some reports have suggested that phenylboronic acid (PBA) has special binding affinity with SA in physiological environment of 7.4 compared to other saccharide.^12–16 Matsumoto et al.¹⁷ used a gold electrode modified with PBA to detect SA at cell membranes in a potentiometric method. Han et al.^14,15 reported that PBA-functionalized QDs could specifically recognize and locate SA on the surface of cells, and cell imaging could be realized by this way. However, up to date, quantitative analysis of SA by PBA-functionalized QDs has not been reported.

Herein, PBA-functionalized Fe₃O₄/CdTe magnetic fluorescence nano probe was designed and prepared. PBA-functionalized Fe₃O₄/CdTe nanoparticles combined SA specifically, and the fluorescence of PBA-functionalized Fe₃O₄/CdTe nanoparticles was lineally quenched by SA. The conjugate could be easily separated from the matrix of samples due to their magnetism. An efficient and specific method to detect SA using PBA-functionalized Fe₃O₄/CdTe nanoparticles as nano probe was built, validated, and finally applied to the determination of SA in infant formula with satisfactory results.

2. Experimental section

2.1 Materials

Tellurium powder (purity > 99%), sodium borohydride, 3-mercapto propionic acid (MPA), sodium hydroxide, ferric chloride hexahydrate (FeCl₃·6H₂O), ferrous chloride tetrahydrate (FeCl₂·4H₂O), ammonia water (NH₃·H₂O 25–28%), PEG400, tetraethylorthosilicate (TEOS), (CH₃COO)₂Cd·2H₂O, PEG400, glucose (Glu), galactose (Gal), mannose (Man) and fructose (Fru) were purchased from Wanqing chemical industry (Nanjing, China). 3-Aminopropyl triethoxysilane (APTES), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC), SA, 3-amino phenylboronic acid (PBA), and dopamine (DA) were purchased from Aladdin industrial corporation (Shanghai, China). Phosphate-buffered saline (PBS, 5 mM, pH 7.4) contained 0.1 M NaCl. The purified water used in the study was prepared using Direct-Q water purification system (Millipore, USA). All chemical reagents and solvents were analytical grade and used without further purification.

2.2 Instruments

FT-IR spectra were recorded by using a Tensor-27 FT-IR spectrometer (Bruker, Germany). The fluorescence emission spectra were measured on F4600 spectrofluorometer (Hitachi, Japan) with 370 nm excitation. UV-vis absorption spectra were obtained on a UV-2450 UV-visible spectrophotometer (Hitachi, Japan). Transmission electron microscopy (TEM) images of the samples were collected on a JEM-1010 transmission electron microscope (Hitachi, Japan). The particle size was determined by dynamic light scattering (DLS) detector (Zs90, Malvern, U. K.). PBA combined on the surface of Fe₃O₄/CdTe nanoparticles was confirmed by gel electrophoresis (Shanghai Tanon HE-120) in 0.5% agarose gel with 0.5× TBE buffer (pH 8.0) at 80 V for 30 min.

2.3 Preparation of amino-modified Fe₃O₄ magnetic nanoparticles

Fe₃O₄ magnetic nanoparticles were prepared through a classic chemical co-precipitation method.^18,19 In order to modify amino groups on Fe₃O₄ magnetic nanoparticles, hydroxyl groups were introduced firstly. PEG400 was added dropwise slowly to Fe₃O₄ suspension at 80 °C under mechanical stirring and N₂ protection. After 60 min, the obtained hydroxyl-modified Fe₃O₄ precipitations were magnetically separated, and washed with purified water and ethanol several times until the pH value reached 7.0. Finally, the hydroxyl-modified Fe₃O₄ precipitations were re-dispersed in 50 ml of ethanol and stored at 4 °C.

The amino-modified Fe₃O₄ magnetic nanoparticles were obtained by adding TEOS and APTES to the above hydroxyl-modified Fe₃O₄ precipitations solution.²⁰ The preparation was described in detail as follows. A suspension of hydroxyl-modified Fe₃O₄ nanoparticles (100 mg) was diluted by a mixture of purified water (20 ml) and ethanol (150 ml), and ultrasonicated for 15 min. Then, 1 ml of NH₃·H₂O (25 wt%) was added. After mechanical stirring at 30 °C for 30 min, 1 ml of TEOS was dropped slowly into the reaction solution. The reaction was continued for 45 min, and then 70 μl of APTES was added to the reaction solution. Finally, the mixture solution was mechanically stirred for another 4 h. The obtained amino-modified Fe₃O₄ magnetic nanoparticles were magnetically separated, washed with purified water and ethanol several times, and finally dried at 30 °C for 4 h.

2.4 Preparation of MPA-caped CdTe QDs

CdTe QDs was synthesized through the reaction between Cd²⁺ and NaHTe solution in a typical synthesis,²¹ using MPA as a stabilizing agent.

2.5 Fabrication of Fe₃O₄/CdTe magnetic fluorescence nanoparticles and PBA-immobilization

The MPA-capped CdTe QDs were conjugated with amino-modified Fe₃O₄ nanoparticles through bonding carboxyl groups on QDs with amino groups on Fe₃O₄ nanoparticles.²² The prepared CdTe QDs were concentrated 10 times, then precipitated in anhydrous ethanol, dissolved again in purified water, and adjusted pH to 7.5. In order to active the CdTe QDs, 25 mg of EDC was added to 1 ml of above CdTe solution, and the mixture was ultrasonicated for 15 min. Then, 1 mg of amino-modified Fe₃O₄ was put into the mixture, and the mixture was ultrasonicated for 5 min, following by vibrating for 4 h at 25 °C. In order to immobilize PBA on the surface of Fe₃O₄/CdTe nanocomposites, 3 mg of PBA was added into the above solution. The mixed solution was vibrated for 1 h at 25 °C. Finally, the obtained PBA-functionalized Fe₃O₄/CdTe magnetic fluorescence nanoparticles were washed thoroughly with PBS (pH 7.4) buffer to remove the excessive CdTe, and stored at 4 °C.

2.6 Fluorescence measurement

0.1 ml of the as-prepared PBA-functionalized Fe₃O₄/CdTe nanoparticles and 0.05 ml of a solution containing SA were added into a centrifuge tube containing 1 ml of PBS (5 mM, pH 7.4) buffer solution. The mixture was incubated at 37 °C for 3 h. Then, the conjugate of PBA-functionalized Fe₃O₄/CdTe nanoparticles and SA was separated by a magnet, washed out with PBS buffer. Finally, the conjugate was dispersed in 1 ml of PBS and its fluorescence intensity was recorded.

2.7 Determination of SA in infant formula

0.3 g of infant formula was dispersed in 20 ml of 0.1 M H₂SO₄, centrifuged at 11 000 rpm at 4 °C for 20 min. The upper cream layer was removed, and the rest was vortexed, and incubated at 80 °C for 2 h. Then, the mixture was cooled with flowing water and centrifuged at 11 000 rpm at 4 °C for 20 min. The supernatant was evaporated, and the residual was dissolved in PBS buffer, and its pH was adjusted to 7.4 with 6 M NaOH. Finally, the volume was set to 4 ml with PBS buffer solution. The fluorescence measurement of the sample was carried out according to Section 2.6.

3. Results and discussion

3.1 Fabrication of PBA-functionalized Fe₃O₄/CdTe nano probe and its working principle for SA detection

Scheme 1 showed the synthesis of PBA-functionalized Fe₃O₄/CdTe nano probe and its working principle for SA detection. In order to obtain Fe₃O₄/CdTe nanoparticles, amino-modified Fe₃O₄ nanoparticles were firstly prepared. Then, Fe₃O₄/CdTe magnetic fluorescence nanoparticles were obtained by binding the amino groups on Fe₃O₄ magnetic nanoparticles with the carboxyl groups on CdTe QDs. Finally, the PBA-functionalized Fe₃O₄/CdTe nano probe was successfully prepared by combining amino groups on PBA with carboxylic groups on Fe₃O₄/CdTe nanoparticles.

Scheme 1 Schematic illustration of synthesis of PBA-functionalized Fe₃O₄/CdTe nano probe and its working principle for SA detection.

As shown in Scheme 1, after incubating SA with the nano probe, PBA on the surface of the probe could covalently combined with cis-diols of SA to form five-membered cyclic complexes, which made the excited electrons from conduction band to valence band hindered.²³ So, the fluorescence intensity of the nano probe decreased with the increasing of SA, which could be attributed to the exciton energy transfer-based fluorescence responses. In addition, the thickness of the capping layer of Fe₃O₄/CdTe nanoparticles was increased and its hydrophilicity was reduced, which also made the fluorescence of the nano probe quenched.²⁴

3.2 Optimization of synthesis conditions

The synthesis of PBA-functionalized Fe₃O₄/CdTe magnetic fluorescence nano probe included several steps. So its fluorescence intensity was affected by some aspects, such as the amount of APTES, EDC and PBA, concentration ratio of reactants, reaction time, pH of QDs, etc. All the influence factors were optimized.

Fig. 1a showed the fluorescence intensity of PBA-functionalized Fe₃O₄/CdTe nano probe under various ratios of QDs to Fe₃O₄. Actually, with the increase of the ratio, the fluorescence intensity of the probe increased due to more CdTe QDs anchoring on the surface of amino-modified Fe₃O₄ nanoparticles. The fluorescence intensity of the probe reached highest and kept almost no changed when the ratio was in the range of 10 : 1–14 : 1. However, when the ratio was higher than 14 : 1, the fluorescence intensity of PBA-functionalized Fe₃O₄/CdTe nano probe became slightly weaker due to the accumulation of QDs. Fig. 1b showed the effect of pH on the fluorescence intensity of PBA-functionalized Fe₃O_4/CdTe nano probe. It indicated that the pH value had obvious influence on the fluorescence intensity of the probe. At low pH value, the fluorescence intensity of the probe was very weak. The reason was that the sulfenly in MPA on the surface of QDs was easily protonized at low pH, which led to the drop of MPA from CdTe QDs.^25,26 It is more difficult for MPA-capped CdTe QDs to conjugate with amino-modified Fe₃O₄ nanoparticles at low pH since the conjugation was undergone through bonding carboxyl groups in MPA on QDs with amino groups on Fe₃O₄ nanoparticles. With the increase of the pH, the fluorescence intensity of the probe increased, and reached the highest value at the pH 7.5. But, when the pH was higher than 7.5, the fluorescence intensity of the probe decreased dramatically. This is because that the activity of EDC as carboxyl activating agent is low at high pH, which made the carboxyl groups on CdTe QDs not adequately activated. So, it was more difficult for MPA-capped CdTe QDs to combine with amino-modified Fe₃O₄ nanoparticles to form the nano probe at high pH. When the pH was higher than 9, the fluorescence of nano probe was slightly recovered. The reason might be that the optimal synthesis condition of CdTe QDs was at pH 9–10 and the fluorescence intensity of QDs was highest at this pH range,²⁷ and this made the fluorescence intensity of the nano probe recovered weakly. Thus, pH value should be carefully controlled when preparing the nano probe.

Fig. 1 Influence of (a) different ratios of MPA-capped QD to amino-modified Fe₃O₄ and (b) pH value on fluorescence intensity of PBA-functionalized Fe₃O₄/CdTe nano probe.

The other influence factors were optimized, and the results were displayed in supplying information.†

3.3 Characterization of Fe₃O₄/CdTe nanoparticles

3.3.1 Infrared spectroscopy. FT-IR spectra of Fe₃O₄, amino-modified Fe₃O₄, Fe₃O₄/CdTe nanoparticles are shown in Fig. 2a–c, respectively. The obvious peaks at 574 cm⁻¹ in Fig. 2a–c corresponded to the stretching vibration of Fe–O bond. The peaks at 1067 and 465 cm⁻¹ in Fig. 2b and c were attributed to the stretching and deformation vibrations of SiO₂, which suggested the successful coating of silica on the surface of the magnetic nanoparticles. In addition, the peak at 1553 cm⁻¹ in Fig. 2b was attributed to the bending vibrations of amino groups, which indicated that amino groups were successfully introduced onto Fe₃O₄ nanoparticles surface. The peak at 1694 cm⁻¹ in Fig. 2c corresponded to the stretching vibration of carboxyl groups, reflecting the existence of carboxyl groups in Fe₃O₄/CdTe nanoparticles since MPA was used to modify QDs. These results showed that CdTe QDs were well conjugated with amino-modified Fe₃O₄ nanoparticles.

Fig. 2 FT-IR spectra of (a) Fe₃O₄, (b) amino-modified Fe₃O₄, (c) Fe₃O₄/CdTe, (d) PBA-functionalized Fe₃O₄/CdTe nano probe, and (e) PBA.

3.3.2 Transmission electron microscopy. Fig. 3 showed TEM images of Fe₃O₄, amino-modified Fe₃O₄ and Fe₃O₄/CdTe nanoparticles. As shown in Fig. 3a, Fe₃O₄ nanoparticles were approximately spherical, and the average size was about 20 nm. After modified with amino groups, the thickness of Fe₃O₄ nanoparticles increased. The mean size of amino-modified Fe₃O₄ nanoparticles was approximately 50 nm according to Fig. 3b. As is well seen, Fig. 3c showed the representative TEM image of Fe₃O₄/CdTe nanoparticles and the average diameter of the nanoparticles was about 80 nm. It confirmed CdTe QDs were well conjugated with amino-modified Fe₃O₄ nanoparticles.

Fig. 3 TEM images of (a) Fe₃O₄, (b) amino-modified Fe₃O₄, (c) Fe₃O₄/CdTe; DLS results of (d) Fe₃O₄/CdTe and (e) PBA-functionalized Fe₃O₄/CdTe nano probe.

3.3.3 Magnetic characteristics. A vibrating sample magnetometer (VSM) was used to measure the magnetic properties of Fe₃O₄, amino-modified Fe₃O₄ and Fe₃O₄/CdTe nanoparticles. As shown in Fig. 4, the saturation magnetization of Fe₃O₄ nanoparticles was 71.37 emu g⁻¹ (Fig. 4a), which was obviously higher than that (45.04 emu g⁻¹) of amino-modified Fe₃O₄ nanoparticles (Fig. 4b) due to the step-by-step fabrication. After amino-modified Fe₃O₄ nanoparticles were combined with non-magnetic CdTe QDs, the measured saturation magnetization of Fe₃O₄/CdTe composite particles was significantly decreased to 9.42 emu g⁻¹ (Fig. 4c).

Fig. 4 VSM magnetization curves of (a) Fe₃O₄ nanoparticles, (b) amino-modified Fe₃O₄ nanoparticles, (c) Fe₃O₄/CdTe nanoparticles. The inset shows photographic images of aqueous solutions of Fe₃O₄/CdTe nanoparticles without a magnetic field (left) and with a magnetic field (right) under 365 nm excitation (1) and normal light (2).

The solutions of Fe₃O₄/CdTe composite particles were placed under normal light (insert in Fig. 4.1) and 365 nm excitation (insert in Fig. 4.2). When a magnet was absent, the colors of the solutions (left side) under normal light and 365 nm excitation were yellow and fluorescent green, respectively. However, when the magnet was placed near the solutions, the magnetic fluorescence nanoparticles dispersed well in the solutions were attracted and concentrated near the magnetic field; both solutions had no colors (right side). All these results showed that Fe₃O₄/CdTe nanoparticles were successfully synthesized.

3.4 Characterization of PBA-functionalized Fe₃O₄/CdTe magnetic fluorescence nano probe

PBA was immobilized on the surface of Fe₃O₄/CdTe nanoparticles to recognize and detect SA. As shown in Fig. 2d of PBA-functionalized Fe₃O₄/CdTe, the IR absorption peak at 1359 cm⁻¹ was assigned to the stretching vibration of B–O bond, which was also observed in PBA (Fig. 2e). The result indicated that PBA was successfully modified on Fe₃O₄/CdTe nanoparticles surface. As shown in Fig. 3d of DLS results, the average diameter of Fe₃O₄/CdTe nanoparticles was about 157.5 nm, which was much bigger than that of TEM results. The reason was that the value under the condition of DLS represented the diameter of aqueous Fe₃O₄/CdTe nanoparticles, not dried nanoparticles. After modified with PBA, the average diameter of Fe₃O₄/CdTe nanoparticles was increased to 215.2 nm (Fig. 3e). The result confirmed the successful conjugation between PBA and Fe₃O₄/CdTe nanoparticles. Fig. 5a–c showed the UV absorption and fluorescence emission spectra of CdTe QDs, Fe₃O₄/CdTe nanoparticles and PBA-functionalized Fe₃O₄/CdTe nano probe, respectively. As shown in Fig. 5b, the absorption spectrum of Fe₃O₄/CdTe nanoparticles was quite broad and shifed toward longer wavelength compared with that of CdTe QDs. Similarly, the red shift was observed in the fluorescence emission spectra of Fe₃O₄/CdTe nanoparticles. Compared with pure CdTe QDs, the fluorescence intensity of Fe₃O₄/CdTe was obviously decreased due to the magnetism of Fe₃O₄/CdTe. These phenomena indicated that the interaction between magnetic nanoparticles and CdTe QDs were taken place. After introducing PBA on the surface of Fe₃O₄/CdTe nanoparticles, the spectra properties had no obvious change due to PBA having little influence on the surface structure of CdTe QDs.

Fig. 5 UV-vis absorption and fluorescence emission spectra of (a) CdTe QDs, (b) Fe₃O₄/CdTe nanoparticles, (c) PBA-functionalized Fe₃O₄/CdTe nano probe. The inset shows agarose gel electrophoresis of (b) Fe₃O₄/CdTe nanoparticles, (c) PBA-functionalized Fe₃O₄/CdTe nano probe.

The agarose gel electrophoresis result showed that PBA-functionalized Fe₃O₄/CdTe nano probe moved slower than the pure Fe₃O₄/CdTe nanoparticles (inset in Fig. 5). It also indicated that PBA was successfully modified on the surface of Fe₃O₄/CdTe nanoparticles.

3.5 Selectivity and stability

The selectivity of PBA-functionalized Fe₃O₄/CdTe magnetic fluorescence nano probe for SA recognition was investigated by comparing SA alone with mixed solution of SA and other interferent. As shown in Fig. 6, the fluorescence intensity kept almost no changed when the mixed samples were added into the solution of the PBA-functionalized Fe₃O₄/CdTe nano probe, compared with that of SA alone. This result showed that PBA-functionalized Fe₃O₄/CdTe nano probe could selectively recognize SA. In addition, the stability of the functional nano probe was also evaluated. In Fig. 7, the fluorescence intensity of the as-prepared nano probe did not change obviously within 12 h. It indicated that the as-prepared nano probe had relatively good stability, suggesting its practical utility.

Fig. 6 Responses of PBA-functionalized Fe₃O₄/CdTe nano probe to SA alone and mixed solution of SA and other interferent: SA (2 mM), Na⁺ (2 mM), K⁺ (2 mM), Mg²⁺ (0.3 mM), Ca²⁺ (0.2 mM), Fe³⁺ (0.02 mM), Zn²⁺ (0.01 mM), Cu²⁺ (1 μM), Hg²⁺ (10 nM), Pb²⁺ (100 nM), Mn²⁺ (1 μM), Vc (0.1 mM), BSA (7.5 mg l⁻¹), DA (0.8 mM), Glu (2 mM), Gal (2 mM), Man (2 mM), Fru (0.8 mM).

Fig. 7 Stability of PBA-functionalized Fe₃O₄/CdTe nano probe.

3.6 Analytical performance of PBA-functionalized Fe₃O₄/CdTe nano probe

Fig. 8 showed the fluorescence emission spectra of PBA-functionalized Fe₃O₄/CdTe magnetic fluorescence nano probe added with different concentrations of SA. As expected, the fluorescence intensity decreased obviously with the increasing concentrations of SA. The decreasing fluorescence intensity was well linear with SA concentrations in the range of 50 μg ml⁻¹–1.50 mg ml⁻¹, and the liner equation is (F₀ − F)F₀ = 0.5617C + 0.0020 with R² of 0.9935 (where, F₀ and F are the fluorescence intensities of PBA-functionalized Fe₃O₄/CdTe nano probe in the absence and presence of SA, respectively, C is the concentration of SA). The limit of detection of 16 μg ml⁻¹ was obtained based on 3σ/k (where, σ is the standard deviation of blank measurements (n = 10), and k is the slope of calibration graph). As shown in Table 1, the intra-day relative standard deviations (RSDs) and the inter-day RSDs of this method were all less than 11.97%. These results suggested that the method exhibited a relatively broader linear range and had good precision.

Fig. 8 Fluorescence spectra of PBA-functionalized Fe₃O₄/CdTe nano probe in the presence of various concentrations of SA (from a to j: 0, 0.05, 0.10, 0.20, 0.40, 0.60, 0.80, 1.00, 1.20 and 1.50 mg ml⁻¹). The insert shows the calibration curve of the SA detection.

Table 1 Precisions of SA determination by PBA-functionalized Fe₃O₄/CdTe nano probe (n = 5)

Concentration of SA (mg ml^−1)	Intra-day (mg ml^−1)	RSD (%)	Inter-day (mg ml^−1)	RSD (%)
0.10	0.10	11.76	0.10	11.97
	0.08		0.12
	0.10		0.12
0.60	0.57	1.74	0.57	6.21
	0.57		0.63
	0.55		0.56
1.20	1.26	0.52	1.26	1.59
	1.26		1.24
	1.24		1.22

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3.7 Analytical application in real sample

To investigate the potential application of the proposed method, the PBA-functionalized Fe₃O₄/CdTe magnetic fluorescence nano probe was applied to the detection of SA in infant formula. The experiment results are shown in Table 2. According to the reference reported, addition levels of SA in infant formula are in the range of 0.8–1.2 mg g⁻¹.²⁸ The result of our detection was 0.82 mg g⁻¹, which coincided with the reference. Moreover, standard addition method was employed to verify the accuracy of the built method. As can be seen, the recoveries of the spiked SA at three different levels in the infant formula samples were from 85.45–92.98% with RSDs less than 8.35%. All the recoveries and RSDs of this method meet the requirements of trace sample analysis. It indicated that the PBA-functionalized Fe₃O₄/CdTe magnetic fluorescence nano probe can be used to monitor SA added in infant formula.

Table 2 Determination of SA in infant formula (n = 5)

Samples	Spiked (mg g^−1)	Found (mg g^−1)	Recovery (%)	RSD (%)
1	0	0.82	—	4.26
2	4.00	4.24	85.45	8.35
3	8.00	7.74	86.51	4.49
4	12.00	11.98	92.98	3.54

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Different methods of SA detection are compared in Table S1.† From the table we can see that the HPLC and fluorescence method have relatively lower LOD. However, the operation of HPLC method needs professional techniques and the fluorescence method in Table S1† also needs expensive reagent such as enzyme, which also makes the protein–QDs not very stable. Although the sensitivity of our method was lower than some of the reported methods,^13,29 it is enough for the present probe to detect SA in infant formula, and it is more simple,rapid and low-costing.

4. Conclusions

In this study, we established a facile method to determine SA using magnetic fluorescence nanoparticles and PBA–SA recognition system. Compared with the reported methods of SA detection, the method built has some advantages: (1) the synthesis of PBA-functionalized Fe₃O₄/CdTe magnetic fluorescence nano probe is facile, low-costing and rapid. (2) This fluorescence nano probe has high selectivity based on PBA–SA recognition system. (3) The conjugate of fluorescence nano probe and SA is easy to be separated from the sample matrix due to the magnetism of the nanoparticles. (4) The PBA-functionalized Fe₃O₄/CdTe magnetic fluorescence nano probe has high stability. So, the present probe has potential application in food quality control.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 81173016), Natural Science Foundation of Jiangsu Education Committee (No. 12KJB350003), and Technology Development Foundation of Nanjing Medical University (No. 2013NJMU023, 2013NJMU024).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra17171g

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