1. Introduction

With the development of nanomaterials ^[1-6],magnetic sensors based onmagnetic nanoparticle (MNPs) for the detection of harmful substances have attracted considerable interest. The magnetic sensors havemany advantages over other sensors. First,a significant improvement in sensitivity can be obtained by this method. Second,different samples can be detected simultaneously by nuclear magnetic resonance instruments,resulting in high throughput analysis. Third,it can be carried out in turbid media due to the elimination of the light scattering requirements common to other methods. In addition,it does not require expensive sophisticated instrumentation and complicated sample preparation processes ^[7-11]. MNPs are important nanomaterials with multiple fundamental properties,including super para-magnetism,high magnetization,high coercivity,and amacroscopic quantumtunneling effect that can be easily modified utilizing other molecules due to the functionalized groups on their surface ^{[12, 13]}. As contrast agents,MNPs have been fabricated in numerous magnetic resonance imaging (MRI) sensors for the detection of harmful elements extending their application to food safety,environmental protection,and many other fields. To date,magnetic sensors have been developed to detect metal ions ^{[7, 8]},small molecules ^{[9, 10]},molecular interactions ^[11],and biological targets such as proteins ^[14-16],bacteria ^{[17, 18]},viruses ^[19],nucleic acids (DNA and mRNA) ^[20-22],and enantiomeric impurities ^[23]. In our previous study,wepresented highly selectivemagnetic sensors forHg²⁺,Cd²⁺ andmelamine based on functionalized Fe₃O₄,(Zn,Mn)Fe₃O₄ and Fe/ Fe₃O₄NPsdue to the special interactionbetween targets andligands,respectively ^[24-26].

Since Pb²⁺ is a common environmental pollutant of high toxicity that can cause renal malfunction and damage to the brain and kidneys and because Pb²⁺ is non-degradable,the accumulation of high levels of Pb²⁺ in children can cause irreversible brain damage,retard mental and physical developments ^[27-29]. In adults,high levels of Pb²⁺ can cause irritability,poor muscle coordination,and nerve damage to the sensory organs ^[27-29]. Moreover,the long-term exposure to low concentrations of Pb²⁺ causes adverse health effect and,as a consequence,the development of ultrasensitive assays for the detection of Pb²⁺ is very important ^[27-29]. Several methods have been used for the detection of Pb²⁺ such as colorimetric sensors ^{[30, 31]},electrochemical sensors ^{[32, 33]},fluorescent sensors ^{[34, 35]},inductively coupled plasma mass spectrometry (ICP-MS) ^[36],atomic absorption spectrometry (AAS) ^{[37, 38]} and inductively coupled plasma atomic emission spectroscopy (ICP-AES) ^[39]. Although these methods are capable of measuring Pb²⁺ with high sensitivity and accuracy,frequently expensive,sophisticated instrumentation and complicated sample preparation processes ^[40] are required. By comparison,the advantages of magnetic sensors have been paid great attention by scientists.

Driven by this need,in our work,we report 3-(3,4-dihydroxyphenyl) propionic acid (DHCA) modified Fe/Fe₃O₄ nanoparticles (DHCA-Fe/Fe₃O₄ NPs) can be used for the detection of Pb²⁺ with excellent selectivity over other metal ions Two phenolic hydroxyls of DHCA can coordinate to the surface of Fe/Fe₃O₄ NPs and by use of the selective coordination interaction between and Pb^{2+ [41]} and the carbonyl groups,Pb²⁺ can induce the assembly of DHCA-Fe/ Fe₃O₄ NPs accompanied by the decrease,then increase in transverse relaxation time (T₂) of surrounding water protons (Scheme 1).

2. Experimental 2.1. Reagents

Iron pentacarbonyl [Fe(CO)₅] was purchased from Development of Beijing Chemical Technology Co.,Ltd. Branch. Oleylamine (70%),oleic acid (90%),1-octadecene (ODE,90%),hexadecylamine (HDA,90%) were obtained from Sigma-Aldrich Shanghai Trading Co.,Ltd. (China). 3-(3,4-dihydroxyphenyl)propionic acid (DHCA) was purchased from Alfa Aesar (China) Chemicals Co.,Ltd. Tetrahydrofuran (THF),NaOH,NH₄F were purchased from Sinopharm Chemical Reagent Co.,Ltd. Deionized water (18.2 MΩ cm) used throughout the experiment was obtained from a Millipore Milli-Q Plus 185 water purification system.

2.2. Apparatus

Transmission electron microscopic (TEM) images of the nanoparticles were observed using a JEOLJEM-2010 Transmission Electron Microscope at 200 kV. Morphological analyses were performed with a Veeco Multimode Ⅲa atomic force microscope (AFM). X-ray diffraction (XRD) was performed using a RigakuDMAX 2000 Diffractometer equipped with Cu/Kα radiation at a scanning rate of 4°/min in the 2θ range from 10° to 80° (λ = 0.15405 nm,40 kV,40 mA). Fourier transforminfrared (FT-IR) spectrawere collected on a Nicolet Avatar 370. The samples were pelletized with KBr for measurements. The hydrodynamic diameter was obtained by dynamic light scattering (DLS) with a Malvern Zetasizer Nano ZS model ZEN3600 (Worcestershire,U.K.) equipped with a standard 633 nm laser. The T₂ value was measured using a 0.5 T NMI 20-CA imaging analyzer (Shanghai Niumag Corporation Limited). The concentration of Fe was obtained by inductively coupled plasma atomic emission spectrometry (ICP-AES) (VistampxicpVarian,USA).

2.3. Synthesis of DHCA-Fe/Fe₃O₄ NPs

The Fe/Fe₃O₄ NPs were synthesized according to the method reported ^[42]. 50 mg of 3-(3,4-dihydroxyphenyl)propionic acid (DHCA) was dissolved in 6 mL of tetrahydrofuran (THF) in a threeneck flask (100 mL). The resulting solution was heated to 50℃ under nitrogen flow. Then,10 mg of Fe/Fe₃O₄ NPs dispersed in 1 mL of THF were added dropwise. After 3 h,the reaction was cooled to r.t.,and 300 mL NaOH (0.5 mol/L) was added to the solution to precipitate the DHCA-Fe/Fe₃O₄ NPs. The precipitate was collected by centrifugation (3000 rpm/min) and redispersed in 2 mL water,then filtered through a 100 nm polycarbonate filter for further use.

2.4. DHCA-Fe/Fe₃O₄ NPs as a magnetic sensor for detection of Pb²⁺

The filtered DHCA-Fe/Fe₃O₄ NPs was diluted 200 times using water ([Fe] = 90 μmol/L). Different volumes of Pb²⁺ (increased from 0 to 140 μL) were added to the diluted solution ([Fe] = 90 μmol/L,3 mL) to obtain various concentrations of Pb²⁺ (0-280 μmol/L). Each combined solution was mixed for 30 min. The T₂ value of the aq. solution of DHCA-Fe/Fe₃O₄ NPs ([Fe] = 90 μmol/L,3 mL) was measured. The relevant parameters were shown as follows. Tw = 6000 ms,SW = 100 kHz,SF = 18 MHz,RG1 = 25db,DRG1 = 3. The change of T₂ value (ΔT₂) was calculated as follows.

$\text{ }\!\!\Delta\!\!\text{ }{{T}_{\text{2}}}\text{=}{{T}_{\text{2}\left( \text{MNPs,P}{{\text{b}}^{\text{2+}}} \right)}}-{{T}_{\text{2}}}\left( \text{MNPs} \right)$

$\text{ }\!\!\Delta\!\!\text{ }{{T}_{\text{2}}}\text{=}{{T}_{\text{2}\left( \text{MNPs,P}{{\text{b}}^{\text{2+}}} \right)}}-{{T}_{\text{2}}}\left( \text{MNPs} \right)$ is the average T₂ of the solution of DHCA-Fe/Fe₃O₄ NPs ([Fe] = 90 μmol/L,3 mL) with a different concentration of Pb²⁺. T_2(MNPs) is the average T₂ of the solution of DHCA-Fe/Fe₃O₄ NPs ([Fe] = 90 μmol/L,3 mL).

2.5. The selectivity of DHCA-Fe/Fe₃O₄ NPs for detection of Pb²⁺

Different volumes of metal ions including Cu²⁺,Zn²⁺,Hg²⁺,Mn²⁺,Mg²⁺,Fe²⁺,Cd²⁺,Ca²⁺,Ni²⁺,Co²⁺,Cr⁶⁺,Al³⁺,Fe³⁺,K⁺,and Na⁺ (increased from 0 to 140 mL) were added to the aq. solution of DHCA-Fe/Fe₃O₄ NPs ([Fe] = 90 μmol/L,3 mL) to obtain various concentrations of metal ions (0-280 μmol/L),respectively. Each combined solution was mixed for 30 min. The competition experiments were performed in the solution of DHCA-Fe/Fe₃O₄ NPs containing 200 μmol/L Pb²⁺ and 200 μmol/L of one of the other metal ions. NH₄F at sixty times the concentration of Al³⁺ and Fe³⁺ was added into the solution of Al³⁺ and Fe³⁺ as masking agents,respectively. The T₂ value was measured using the same parameters in Section 2.4.

3. Results and discussion 3.1. Synthesis and characterization of DHCA-Fe/Fe₃O₄ NPs

The Fe/Fe₃O₄ NPs were synthesized according to the method reported ^[42]. The synthesized Fe/Fe₃O₄ NPs showed excellent dispersability in hexane with an average diameter of 13.3 ± 1.5 nm (Fig. 1a and inset). The iron oxide shell prevented the iron core from further oxidation. The XRD pattern confirmed the existence of bcc-Fe NPs with the characteristic peaks of (1 1 0) and (2 0 0) planes (Fig. S1 in Supporting information),however,no Fe₃O₄ peaks can be seen due to the peak broadening of their small crystal domains. To render Fe/ Fe₃O₄ NPs with the detection capability in aq. medium,DHCA was modified on the surface of nanoparticles by the ligand exchange method ^[43]. As shown in the TEM image,the mean diameter of DHCA-Fe/Fe₃O₄ NPs was 13.0 ± 1.0 nm (Fig. 1b and inset). Due to the low self-aggregation by magnetic interactions among NPs,the hydrodynamic diameter of DHCA-Fe/Fe₃O₄ NPs was ~22 nm (Fig. 1c). In order to indicate DHCA was successfully bound on Fe/ Fe₃O₄ NPs,the FT-IR spectra of DHCA,Fe/Fe₃O₄ NPs and DHCA-Fe/ Fe₃O₄ NPs were investigated. After the ligand exchange reaction,the characteristic peaks of the benzene ring and COO^-,-C-O-,-C-H- stretching vibration of DHCA-Fe/Fe₃O₄ NPs were observed at 1553 cm^-1 and 1481 cm^-1,1385 cm^-1,1253 cm^-1,806 cm^-1,respectively,indicating that DHCA had successfully modified the surface of Fe/Fe₃O₄ NPs (Fig. 1d).

3.2. DHCA-Fe/Fe₃O₄ NPs as a magnetic sensor for detection of Pb²⁺

The good magnetic property of DHCA-Fe/Fe₃O₄ NPs should be well suitable for a magnetic sensor ^[42]. With the addition of Pb²⁺ to an aq. solution of DHCA-Fe/Fe₃O₄ NPs,in which the concentration of Fe determined by ICP-AES was~90 μmol/L,a decrease,then increase in ΔT₂ was obtained with the enhancement of Pb²⁺ within the concentration of 280 μmol/L (Fig. 2a). With the concentration of Pb²⁺ below 120 μmol/L,the aggregated DHCA-Fe/Fe₃O₄ NPs was so small that the ΔT₂ was decreased. With the concentration of Pb²⁺ from 120 μmol/L to 220 μmol/L,the aggregated DHCA-Fe/ Fe₃O₄ NPs was large enough,so the ΔT₂ was increased. With the concentration of Pb²⁺ from 40 μmol/L to 100 μmol/L and from 130 μmol/L to 200 μmol/L,the ΔT₂ has a linear relationship with Pb²⁺,respectively. The correlation coefficient (R² = 0.99825 and R² = 0.98765) were obtained (Fig. 2b and c). Therefore,the quantitative detection of Pb²⁺ can be performed in the concentration range of Pb²⁺ from 40 μmol/L to 100 μmol/L and from 130 μmol/L to 200 μmol/L. Upon addition of the concentration of Pb²⁺ above 220 μmol/L to an aq. solution of DHCA-Fe/Fe₃O₄ NPs ([Fe] = 90 μmol/L,3 mL),the rate change of ΔT₂ slowed. The possible reason for the change of ΔT₂ was as follows: At a concentration below 220 μmol/L of Pb²⁺,the binding sites of DHCA-Fe/Fe₃O₄ NPs exceeded those of Pb²⁺,which resulted in ΔT₂ greatly changed; when the concentration of Pb²⁺ was above 220 μmol/L,the binding sites of Pb²⁺ were more than the saturated binding sites of DHCA-Fe/Fe₃O₄ NPs and the addition of Pb²⁺ did not combine with DHCA-Fe/Fe₃O₄ NPs completely. Therefore,the ΔT₂ would obviously not change with the addition of Pb²⁺.

3.3. The mechanism of Pb²⁺ detection

To investigate the mechanism of Pb²⁺ detection, the changes of the hydrodynamic diameter of an aq. solution of DHCA-Fe/Fe₃O₄ NPs were monitored upon addition of the different concentrations of Pb²⁺. The hydrodynamic diameter of an aq. solution of DHCA-Fe/ Fe₃O₄ NPs ([Fe] = 90 μmol/L,3 mL) increased from ~22 nm to ~1720 nm with the increase of the concentration of Pb²⁺ from 0 to 280 μmol/L (Fig. 3a),which resulted from the aggregation induced by the coordination interaction between DHCA and Pb²⁺ (Scheme 1). The hydrodynamic diameter of an aq. solution of DHCA-Fe/Fe₃O₄ NPs was ±22 nm,with this sufficiently small DHCA-Fe/Fe₃O₄ NPs,the diffusional motion of water molecules is fast enough to average out the magnetic fields produced by DHCAFe/ Fe₃O₄ NPs,which should be in the motional averaging (MA) regime. At a concentration of 120 μmol/L Pb²⁺,the hydrodynamic diameter of the aq. solution of DHCA-Fe/Fe₃O₄ NPs was ±332 nm which exceeded the traveling distance of diffusing water molecules. In this corresponding regime,namely the static dephasing (SD) regime ^[44],the averaging effect disappears and DHCA-Fe/Fe₃O₄ NPs act as randomly distributed stationary molecules. In this type of change between MA and SD regimes,DHCA-Fe/Fe₃O₄ NP aggregation induced by Pb²⁺ caused a T₂ decrement. With the concentration of Pb²⁺ above 120 μmol/L,the hydrodynamic diameter of the DHCA-Fe/Fe₃O₄ NPs increased beyond the SD regime,the T₂ began to increase as the system moved into the echo-limited (EL) regime. DHCA-Fe/Fe₃O₄ NPs became so large that many water protons failed to undergo magnetic field inhomogeneity,and they showed a high average T₂ value just like free water molecules without having any magnetic interaction. In this type of change between SD and EL regimes,DHCA-Fe/Fe₃O₄ NP aggregation induced by Pb²⁺ caused a T₂ increment. Therefore,the changes of T₂ value were attributed to the increase of the size of DHCA-Fe/Fe₃O₄ NPs induced by different concentration of Pb²⁺. The TEM image of DHCA-Fe/Fe₃O₄ NPs further confirmed this point. In the presence of 240 μmol/L Pb²⁺,the significant aggregation with an irregular shape was clearly observed (Fig. 3b). The aggregation behavior of the magnetic sensor with Pb²⁺ ion was also confirmed by AFM images. As shown in Fig. 3c and d,many disperse and bright spots were seen on the background in the solution of DHCA-Fe/Fe₃O₄ NPs. On the contrary,in the presence of 240 μmol/L Pb²⁺,the aggregated spots were obtained.

3.4. The selectivity of DHCA-Fe/Fe₃O₄ NPs for detection of Pb²⁺

The specificity of DHCA-Fe/Fe₃O₄ NPs ([Fe] = 90 μmol/L) was tested by using othermetal ions in place of Pb²⁺ ,including Cu²⁺,Zn²⁺,Hg²⁺,Mn²⁺,Mg²⁺,Fe²⁺,Cd²⁺,Ca²⁺,Ni²⁺,Co²⁺,Cr⁶⁺,Al³⁺,Fe³⁺,K⁺,and Na⁺ at various concentration (0-280 μmol/L). Nearly no obvious changes of ΔT₂ were observed,except Al³⁺ and Fe³⁺ (Fig. S2 in Supporting information). SinceNH₄Fwill be able to formmuchmore stable complexes with Al³⁺ and Fe³⁺ than with othermetal ions,and not be able to link with Fe/Fe₃O₄ NPs,thus it can suppress interference of Al³⁺ and Fe³⁺ with thenanoparticle surface.Therefore,NH₄F was used to mask Al³⁺ and Fe³⁺. With the addition of NH₄F to the solution of Al³⁺ and Fe3+,respectively,the changes of ΔT₂ were negligible (Fig. 4a). To further confirmthe effect by other metal ions,the competition experiments was done,in which Pb²⁺ was mixed with one of the other metal ions and NH₄F was also added into the solution of Al³⁺ and Fe³⁺,respectively. No significant variation in the ΔT₂ was observed (Fig. 4b). The resultwas perhaps attributed to the special coordination interaction between DHCA and Pb^{2+ [35]}.

4. Conclusion

In conclusion,a novel method for the highly sensitive detection of Pb²⁺ was established based on the aggregation of magnetic nanoparticles induced by the coordination interaction between DHCA and Pb²⁺. In addition,excellent selectivity was confirmed by the detection of other metal ions. This method exhibits a simple,rapid and efficient procedure for the detection of Pb²⁺,and can be widely applied in the detection of other species as a general technology based on functionalized magnetic nanoparticles.

Appendix A. Supplementary data

Supplementarymaterial related to this article canbe found,inthe online version,at http://dx.doi.org/10.1016/j.cclet.2016.01.060.