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Development of a novel tridentate ligand for colorimetric detection of Mn2+ based on AgNPs
Accepted Manuscript Development of a novel tridentate ligand for colorimetric detection of Mn2+ based on AgNPs
Jianyu Wei, Jinfan Chen, Guozong Yue, Liangsheng Hu, Danqing Zhao, Jing Zhu, Luming Yang, Deshun Huang, Pengxiang Zhao PII: DOI: Reference:
S1386-1425(18)30424-4 doi:10.1016/j.saa.2018.05.033 SAA 16067
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received date: Revised date: Accepted date:
8 February 2018 3 May 2018 8 May 2018
Please cite this article as: Jianyu Wei, Jinfan Chen, Guozong Yue, Liangsheng Hu, Danqing Zhao, Jing Zhu, Luming Yang, Deshun Huang, Pengxiang Zhao , Development of a novel tridentate ligand for colorimetric detection of Mn2+ based on AgNPs. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Saa(2017), doi:10.1016/j.saa.2018.05.033
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ACCEPTED MANUSCRIPT Development of a novel tridentate ligand for colorimetric detection of Mn2+ based on AgNPs Jianyu Weia,b, Jinfan Chena,Guozong Yuea, Liangsheng Huc, Danqing Zhaoa, Jing Zhua, Luming Yangb, Deshun Huanga* and Pengxiang Zhaoa* a
Institute of Materials, China Academy of Engineering Physics, No. 9, Huafengxincun, Jiangyou City, Sichuan
b
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Province, 621908, P. R. China. Email: [email protected]; [email protected] The Key Laboratory of Leather Chemistry and Engineering of Ministry of Education, Sichuan University,
c
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Chengdu, 610065, P. R. China.
Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of
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Guangdong Province, Shantou University, Guangdong, 515063, P. R. China
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Abstract
A novel tridentate ligand nitrilotris(methylene)tris(1,2,3-triazole)triacetate (NTTTA) has been
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synthesized by click reaction and followed with ester hydrolysis reaction. The silver nanoparticles (AgNPs) were then modified and stabilized by this ligand, and subsequently been employed for the
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highly selective and sensitive colorimetric detection of Mn2+ in aqueous solution. The presence of
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Mn2+ can cause the aggregation of AgNPs, which leads to the color change of the dispersion from yellow to brown, as well as the decrease and red-shift of the surface plasmon resonance absorption. The detection limit of Mn2+ was as approximately 0.5 µM by the naked eyes. UV-vis spectroscopy
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analysis showed a good linear relationship between the logarithm of the ratios (A550/A395) and the concentration of Mn2+over the range of 0.05 µM-10 µM, and the LOD was calculated to be 12.6
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nM (S/N = 3). The present assay showed good simplicity without the need of adjusting the pH value. The feasibility of this technique was evaluated for successful detection of Mn2+ in tap water and lake water samples, with good recoveries.
Keywords: tridentate ligand; AgNPs; colorimetric detection; Mn2+
1. Introduction Colorimetric sensing based on gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) has attracted remarkable attention because of its simplicity, rapidity, and high sensitivity and selectivity [1]. AuNPs and AgNPs are excellent candidates for the fabrication of novel chemical and 1
ACCEPTED MANUSCRIPT biological sensors because of the unique physical and chemical properties [2,3]. Firstly, they possess high surface-to-volume ratio with ease of modification through suitable surface chemistry. Secondly, their localized surface plasmon resonances (LSPR) related optical properties can be readily tuned by varying their size, shape, and the surrounding chemical environment. Finally, their molar extinction coefficients were much greater compared to that of common organic dyes, which may
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increase the detection sensitivity when used in absorption spectroscopy [4]. It is noting that the molar extinction coefficient of AgNPs is ~100 fold greater than that of
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AuNPs. Therefore, as a sensor, AgNPs somehow should have a big advantage compared with AuNPs. Therefore, many colorimetric methods have been reported for detection of heavy metal ions
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based on gold or silver nanoparticles[4-6]. For an example, manganese ion (Mn2+) is one of the essential microelements in human body, which plays important roles in life processes[7]. However,
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excessive intake of Mn2+ is harmful, which may cause functional handicaps and some disorders to the endocrine, nervous and reproductive systems[8]. Therefore, much attention has been attracted
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to develop methods for detecting Mn2+ with high sensitivity and selectivity. Apart from conventional Mn2+ detection methods such as atomic absorption spectrometry (AAS) [9,10], inductively coupled
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plasma mass spectroscopy (ICP-MS) [11], and inductively coupled plasma-atomic emission
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spectrometry (ICP-AES)[12], colorimetric assay based on AuNPs and AgNPs has emerged as an more economical method. Colorimetric detections of Mn2+ based on AuNPs were reported using polyacrylamide (PAM) [13] or L-dopa[14] as modifiers with the detection limit of 5 µM. While
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those based on AgNPs were reported using clove (S. aromaticum) [15], L-arginine[16], P2O74-[17], 4-mercaptobenzoic acid (4-MBA) and melamine (MA) (4-MBA-MA) [18], P3O105-[19], β-
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cyclodextrin-adamantane (β-CD-ADM) [20], Na4P2O7 and hydroxypropylmethylcellulose (Na4P2O7-HPMC) [21] or cysteic acid (CA)[22] as modifiers, with the lowest detection limit of 20 nM. Although these methods have led to successful development of Mn2+sensors, they always need to adjust pH values which reduced the simplicity, and some of the detection limit is above the recommended limit of the US Environmental Protection Agency for drinking water (0.9 µM). Recently, we have reported a highly efficient way to detect the Cr(III) in aqueous solution by nitrilotriacetate (NTA) modified AuNPs[23]. It is disclosed that Cr(III) selectively coordinated with NTA on the AuNPs surface that led the aggregation of AuNPs. As well known, the surface modifiers have great impact on the sensitivity and selectivity of AuNPs or AgNPs.We suppose that some 2
ACCEPTED MANUSCRIPT modification of the NTA ligand could change the selectivity from Cr(III) to other metal ions.In this report, an analogue of NTA, a novel tridentate ligand nitrilotris(methylene)tris(1,2,3triazole)triacetate (NTTTA) has been synthesized through “click” formation of 1,2,3-triazole and the following ester hydrolysis reaction. The as prepared ligand has been used as a modifier of AgNPs to form the NTTTA-AgNPs assay, which showed highly sensitive and selective colorimetric
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detection of Mn2+ in aqueous solution, without the need of adjusting the pH value. Mechanism studies suggested that the 1,2,3-triazole play important role in changing the selectivity from Cr(III)
2. Results and discussion 2.1 The preparation and characterization of NTTTA-AgNPs
synthetic
procedure
of
the
tridentate
ligand
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The
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to Mn2+, through 1,2,3-triazole-carboxy-Mn coordination.
nitrilotris(methylene)tris(1,2,3-
triazole)triacetate (NTTTA) stabilized AgNPs was shown in Scheme 1. Firstly, tripropargylamine
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(1) and ethyl 2-azidoacetate (2) were synthesized by common nucleophilic substitution reactions respectively. Then 1 and 2 underwent the Cu-catalyzed alkyne-azide cycloaddition (CuAAC) (“click”
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reaction) [24] to form the triazole compound 3. The compound 3 was then hydrolyzed in NaOH
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solution and subsequently acidified to obtain the NTTTA-acid, compound 4. The obtained NTTTAacid was characterized using nuclear magnetic resonance spectrum (NMR) (see supporting information), high resolution mass spectrometry (HRMS). Finally, the target NTTTA , compound 5
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was generated by basification of the NTTTA-acid using three equivalents of NaOH. In order to the prepare the NTTTA-AgNPs assay, unmodified AgNPs were firstly synthesized using NaBH4, and
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subsequently capped with the NTTTA ligand (Scheme 1).
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Scheme 1 The preparation of NTTTA stabilized AgNPs.
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The obtained NTTTA-AgNPs appeared a pale yellow color which was stored in a refrigerator
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at 4 oC before use. UV-vis. spectroscopy indicated a maximum absorption wavelength at 395 nm, which was slightly red-shifted compared with the unmodified AgNPs (390 nm) (Fig. S1†). The
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TEM images indicated that the NTTTA-AgNPs were well dispersed in water with an average
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particle size of 11 nm (Fig. 1A and S2†). However, after mixing with 10 µM Mn2+ ions, aggregation
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of NTTTA-AgNPs happened which could be shown in the TEM images (Fig 1B).
Fig. 1 TEM images of NTTTA-AgNPs before (A) and after (B) mixing with Mn2+ (10 µM).
2.2 Optimization of experimental conditions
In order to investigate the effect of the NTTTA concentration on the selectivity and sensitivity of the sensor, several NTTTA-AgNPs with different molar ratios of NTTTA ligand and AgNO3
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of the NTTTA-AgNPs assay for the Mn2+ detection increased greatly. However, if the NTTTA/AgNO3 ratios were too small (0.25 and 0.5), some other metal ions like Cu2+ and Cd2+
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sensitivity, the NTTTA/AgNO3 ratio was optimized to be 1.
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interfered the Mn2+ detection (Fig. 2B). Consequently, on the consideration of both selectivity and
Fig. 2 (A) The effect of NTTTA/AgNO3 ratios on the stability and sensitivity of NTTTA-AgNPs.
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(B) Photographic images of NTTTA-AgNPs in the presence of Mn2+ (10 µM) or other metal ions (30 µM), with NTTTA/AgNO3 ratios of 0.25 (I), 0.5 (II), 1 (III), and 2 (IV).
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Subsequently, the incubation time of the probe was also tested for the colorimetric detection of Mn2+. In the presence of 10 µM Mn2+, the A550/A395 ratio increased rapidly within 6 minute (Fig. S3 †). Further increase of time led to slow and little increase of the ratio. Therefore, 6 minute was selected as the optimized incubation time for further detection.
2.3 Selectivity for Mn2+ detection with NTTTA-AgNPs
In order to demonstrate the selectivity for Mn2+ detection with NTTTA-AgNPs system, eye vision and UV-vis spectra of the assay after adding of Mn2+ (10 µM) or other ions including Fe3+, Fe2+, K+, Al3+, Cu2+, Cd2+, Co2+, Ni2+, Pb2+, Zn2+, Hg2+, Mg2+, Ca2+, Cr3+, Cr2O72-, NO3-, PO43-, CO32-, 5
ACCEPTED MANUSCRIPT SO42-, Br-, NO2-, SCN-, and HCOO- (30 µM respectively) were investigated. As shown in Fig. 3, only the presence of Mn2+ (10 µM) led to an apparent decrease and red-shift of extinction band (Fig. 3A), as well as the color change from pale-yellow to brown (Fig. 3B), whereas the other types of ions had slight changes even at much higher concentration (3 times of Mn2+). Moreover, the A550/A395 ratio of NTTTA-AgNPs after mixing with Mn2+ was considerably larger than the ratios of
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which mixing with other ions (Fig. 3B).
Fig. 3 (A) UV-vis spectroscopy of NTTTA-AgNPs and (B) A550/A395 ratios and photographic image (inset) of NTTTA-AgNPs after the addition of Mn2+ (10 µM) and other ions (30 µM respectively).
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The influence of the presence of other ions towards the detection of Mn2+ was also investigated.
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It is obvious the color and A550/A395 ratios of the NTTTA-AgNPs incubated with the mixture of Mn2+ and other ions (Fig. 4) were all similar with those only incubated with Mn2+ (Fig. 3B). These results indicated that the presence of other ions had little interference on the Mn2+ detection because
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they cannot inhibit the aggregation of the AgNPs induced by Mn2+.
Fig. 4 A550/A395 ratios and photographic images (inset) of NTTTA-AgNPs incubated with Mn2+ (10 µM) and another ions (10 µM, respectively). 6
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2.4 Sensitivity forMn2+ detection with NTTTA-AgNPs
The sensitivity of the NTTTA-AgNPs assay towards Mn2+ detection was conducted by eye vision and UV-vis spectra under optimized conditions. As shown in Fig. 5, with the increase of the Mn2+ concentration (from 0.05µM to 20 µM), the color of the NTTTA-AgNPs changed gradually
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from pale-yellow to brown. The detection limit of Mn2+ was approximately 0.5 µM by eye vision. The UV-vis spectra of AgNPs shown in Fig. 5A indicated that, with the increase of Mn2+
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concentration, the LSPR absorption at 395 nm gradually decreased, while the red-shifted absorption band at 550 nm gradually increased. Fig. 5B showed the relationship between logarithm of the
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A550/A395 ratios of different AgNPs dispersions and concentrations of Mn2+ ranging from 0.05 to 20 µM. As shown in the inset of Fig. 5B, a good linear relationship (Y=-1.20183+0.05377X) was
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obtained over the range of 0.05 μM-10 μM with the correlation coefficient of 0.99283, and the LOD was 12.6 nM (S/N = 3) by UV-vis spectroscopy. Comparing with the reported colorimetric sensors,
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the current reported NTTTA-AgNPs system showed simple and high sensitivity without pH control
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(Table 1).
Fig. 5(A) UV-vis spectroscopy and photographic images (inset) of NTTTA-AgNPs after mixing with different concentration of Mn2+ (0.05-20 µM). (B) Plot of lg(A550/A395) as a function of the concentrations of Mn2+ and the linear calibration plot for the concentrations of Mn2+ from 0.05 to 10 µM (inset). Table 1. Comparison of various colorimetric sensors for the detection of Mn2+. Detection system
pH
LOD
Incubation time
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ref
-
5 µM (eye)
10 min
[14]
PAM-AuNPs
4.5
5 µM (S/N = 3)
2 min
[13]
AgNPs-L-arginine
9.4
20 nM (S/N = 3)
40 min
[16]
P2O74--AgNPs
~10
0.03 µM (S/N = 3)
20 min
[17]
4-MBA-MA-AgNPs
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50 nM (S/N = 3)
5 min
[18]
clove-AgNPs
11.5
0.2 µM (eye)
30 min
[15]
P3O105−-AgNPs
~10.8
0.1 µM (eye)
Several min
[19]
β-CD-ADM AgNPs
-
0.5 µM (UV-vis)
5 min
[20]
Na4P2O7-HPMC-AgNPs
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0.5 µM (eye)
10 min
[21]
CA-AgNPs
7.9
5 nM (UV-vis)
30 s
[22]
NTTTA-AgNPs
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0.5 µM (eye)
6 min
This work
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L-dopa-AuNPs
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12.6 nM (S/N = 3)
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2.5 Detection of Mn2+ in real water samples with the NTTTA-AgNPs assay
The feasibility of the current NTTTA-AgNPs assay has been evaluated by subjecting the real
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water samples (tap water and lake water), which were spiked with various concentrations of Mn2+
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standard solution, to the detection system. The results disclosed that the concentrations determined with the current method (third column in Table 2) were very close to the real spkied concentrations of the samples (second column in Table 2). Therefore, the NTTTA-AgNPs assay could be realistic
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for the determination of Mn2+ in tap water and lake water samples. Table 2. The results of the detection of Mn2+ in real water samples with the NTTTA-AgNPs assay.
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Samples
Tap water
Lake water
Spiked (μM)
Found (n=3) (μM)
Recovery (%)
RSD/%
0.50
0.544
108.8
7.97
2.0 6.5
1.981 6.466
99.05 99.48
0.91 0.21
1.0
1.016
101.6
1.31
3.0
3.034
101.13
1.2
7.0
7.042
100.6
0.42
2.6 The mechanism of Mn2+ detection
Basically, the as-prepared NTTTA-AgNPs were modified and stabilized by the electrostatic
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ACCEPTED MANUSCRIPT repulsion between the negatively charged NTTTA ions and the coordination between the tridentate NTTTA ligands and the AgNPs surface. Form the UV-vis. spectroscopy and TEM images (Fig. S1 † and 1), it can be concluded that the colorimetric detection of Mn2+ was based on the aggregation of NTTTA-AgNPs. We further studied the aggregation mechanism using FT-IR spectra and Raman spectra analysis.
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FT-IR spectra of NTTTA, NTTTA-AgNPs in the absence and presence of Mn2+ were firstly characterized and shown in Fig. 6. The FT-IR spectra of NTTTA showed strong peaks at 1617 cm-1
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and 1391 cm-1 which represented the carboxylate groups. The FT-IR spectra of NTTTA-AgNPs showed similar peaks at 1627 cm-1 and 1384 cm-1 which indicated the successful modification of
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NTTTA on the AgNPs surface. Then, in the presence of Mn2+, the peak at 1627 cm-1 shifted to 1631
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cm-1, which suggested coordination between the carboxylate group of NTTTA and Mn2+.
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Fig. 6 FT-IR spectra of NTTTA, NTTTA-AgNPs, and NTTTA-AgNPs in the presence of Mn2+. Subsequently, Raman spectra of NTTTA, NTTTA-AgNPs, NTTTA-AgNPs in the presence of
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Mn2+ with different concentrations were also recorded. As shown in Fig. 7, the Raman signal of NTTTA alone could not be detected. However, the NTTTA-AgNPs exhibited strong surfaceenhanced Raman scattering (SERS) effect and yielded strong vibrational signals of the NTTTA ligand, which further indicated the successful modification of NTTTA on the AgNPs surface[25]. In addition , the Raman peak at 1561 cm-1 could be contributed to the N=N stretching of the 1,2,3triazole group. It could be seen from Fig. 7, this peak was largely enhanced in the presence of increasing concentration of Mn2+, accompanied with slight blue-shift. These changes could be attributed to binding of Mn2+[26], which suggested the coordination between the 1,2,3-triazole group of NTTTA and Mn2+.
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Fig. 7 Raman spectra of NTTTA, NTTTA-AgNPs, and NTTTA-AgNPs in the presence of Mn2+(0.5 μM and 6 μM).
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As we known, metal ions are prone to coordinate with the 1,2,3-triazole to form five or sixmembered cycles, through bi-dentate coordination with both the donor site adjacent to N1 and the
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N2 site on the 1,2,3-triazole [27]. Based on the FT-IR spectra and Raman spectra analysis, it could be concluded that in the presence of Mn2+, the Mn2+ could be preferred to form inter-molecular
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coordination with two branches of NTTTA ligands on the AgNPs surface, through bi-dentate
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coordination with the carboxyl oxygen atom adjacent to N1 and the N2 atom on the 1,2,3-triazole,
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which cause cross-linking of AgNPs and leading the aggregation (Scheme 2).
Scheme 2 Possible mechanism of the NTTTA-AuNPs based colorimetric detection of Mn 2+. We then performed density functional theory (DFT) calculations to further explain the coordination mode between NTTTA and Mn2+. The change of the energy (ΔE) of the coordination
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ACCEPTED MANUSCRIPT interactions between Mn2+ and NTTTA ligand based on the above coordination mode can be calculated by the following formula 34Mn(H2O)62 2NTTTA Mn(NTTTA) 2(H2O)2 4H2O
(1)
That is, the hexahydrated Mn2+ ion underwent ligand exchange with two NTTTA molecules, forming a hexacoordinate structure with the Mn2+ ion serving as the coordination core while the
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carboxyl oxygen atom and the N2 atom on the 1,2,3-triazole from two NTTTA molecules and two water molecules coordinate to the core (Fig. 8). Based on optimized configuration, the ΔE was
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calculated to be -305.9 kcal/mol, which indicates that the proposed coordination mode is very
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energetically favorable.
3. Experimental
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Fig. 8 The calculated coordination model between Mn2+ and NTTTA ligand.
3.1 Materials and instrumentation
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All commercially available reagents were used without any further purification. K2Cr2O7 was used to prepare the Cr2O72- standard solution. PbNO3 was used to prepare the Pb2+ standard solution.
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Chloride salts of other metal salts were used to prepare the standard solution of other metal ions. NaNO3, Na3PO4, Na2CO3, Na2SO4, KBr, NaNO2, KSCN, and HCOONa were used to prepare the standard solution of respective anions. Glassware was washed with aqua-regia (HCl:HNO3=3:1). Ultra-pure water was used in all the experiments unless otherwise stated. UV-vis spectroscopy measurements were carried out by a UV-1800 SHIMADZU spectrometer. FT-IR spectra were obtained by a Nicolet-iS10 instrument. 1H and 13C NMR spectroscopy measurements were carried out by using a Bruker Avance 300 Spectrometer. HRMS spectra measurements were obtained through an AxION 2 TOF Mass Spectrometer. The TEM images were acquired by a STEM (JEOL JEM-2100F) instrument operated at 200 kV. Raman spectra were obtained on a Renishaw 2000 11
ACCEPTED MANUSCRIPT instrument. 3.2 The preparation of NTTTA NTTTA was synthesized by Cu-catalyzed alkyne-azide (CuAAC) reaction using tripropargylamine (1) and ethyl 2-azidoacetate (2), and followed by hydrolysis reaction in the presence of NaOH. Firstly, tripropargylamine (1) and ethyl 2-azidoacetate (2) were prepared using
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the following procedure. To the stirred solution of CH2Cl2 (50 mL) were added 3-bromopropyne (6.248 g, 42 mmol, 80%
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wt in toluene), 2-propynylamine (1.1016 g, 20 mmol) and NaOH (2 g, 50 mmol). After stirring at
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room temperature for 5 days, the resulting suspension was diluted with water (20 mL) and extracted by CH2Cl2 (2×20 mL).The organic layer was washed with water (20 mL), dried over anhydrous
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Na2SO4, filtered, and concentrated under reduced pressure to get the compound 1. 1H NMR (300 MHz, CDCl3) δ 3.48 (d, J = 2.4 Hz, 6H), 2.26 (t, J = 2.4 Hz, 3H).
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Ethyl bromoacetate (3.34 g, 20 mmol) and sodium azide (1.56 g, 24 mmol) were added into a mixture solution of H2O (10 mL) and acetone (30 mL). After stirring at room temperature for 5h,
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the resulting solution was diluted with water (20 mL) and extracted by CH2Cl2 (2×20 mL). The
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organic layer was washed with water (20 mL), dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure to get the compound 2. 1H NMR (300 MHz, CDCl3) δ 4.27 (q, J = 6.0 Hz, 2H), 3.86 (s, 2H), 1.31 (t, J = 6.0 Hz, 3H).
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Accordingly, to the stirred mixture of compound 1 (3.87 g, 30 mmol) and 2 (1.31 g, 10 mmol) in THF (80 mL) were added the aqueous solution of CuSO4 (30 mL, 0.17 M) and sodium ascorbate
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(30 mL, 0.3 M) at room temperature under nitrogen atmosphere. The resulting solution was stirred overnight at room temperature. After removal of THF in vacuo, CH2Cl2 (30 mL) and aqueous EDTA (30 mL, 0.2 M) were added to the reaction mixture, which was stirred for 30 min to remove the Cu ions. The organic layer was separated, and the water phased was extract with CH2Cl2 (30 mL). The organic layer was collected, washed with water (30 mL), dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure to get the crude compound 3, which was subjected to the hydrolysis reaction directly. To the stirred solution of compound 3 in methanol (30 mL) was added NaOH (1.22 g, 30.5 mmol) under room temperature. After stirring for 3h, aqueous HCl (1 M) was added to adjust the pH value to approximately 3.0. The resulting mixture was concentrated under 12
ACCEPTED MANUSCRIPT reduced pressure to the obtained the crude product which was purified by silica column chromatography (EtOAc/MeOH, 1:1 to 1:5) to obtain the target NTTTA-acid (4) (acid form) as pale yellow solid. 1H NMR (300 MHz, D2O) δ 8.18 (s, 3H), 5.06 (s, 6H), 4.49 (s, 6H); 13C NMR (75.4 MHz, D2O) 172.95, 136.79, 128.67, 55.18, 46.46. HRMS (ESI) Calcd for C15H17N10O6 (M-H): 433.1338; Found: 433.1348.
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3.3 The preparation of NTTTA-AgNPs NTTTA (5) (0.048 M) was firstly prepared by mixing the above synthesized NTTTA-acid with
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three equivalents of NaOH in aqueous solution. AgNPs were prepared by reducing AgNO3 with NaBH4. For details, NaBH4 (5 mg, 0.132 mmol) was rapidly added into the aqueous solution of
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AgNO3 (50 mL, 1×10-4 M) under vigorously stirring at room temperature.The resulting solution was stirred for 10 min, during which time the color changed from colorless to pale-yellow. Subsequently,
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aqueous solution of NTTTA (0.104 mL, 0.048 M) was added quickly into the above solution with continuous stirring for another 30 min. The obtained NTTTA-AgNPs was stored in a refrigerator at
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4oC and subjected to the detection process directly without further purification. 3.4 Colorimetric Detection of Mn2+ using NTTTA-AgNPs
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Mn2+ aqueous solution (0.1 mL) of certain concentrations was homogeneous mixed with the
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as prepared NTTTA-AgNPs (0.9 mL), which was incubated at room temperature for 6 min.Then the photographic images and UV-vis spectroscopy measurements were recorded. 3.5 Computational details
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Spin-polarized DFT simulations were carried out using the PWscf module implemented in the plane-wave Quantum_Espresso package[28] to study the coordination reactions. The molecular
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cluster was situated in a cubic unit cell with the side-length of 35 Å. Gamma-point sampling was adopted and the Makov-Payne method[29] was used to obtain the correction to the total energy due to the use of the periodic conditions in the calculations. The generalized gradient approximations (GGA) Perdew-Burke-Ernzerhof (PBE)[30] functional, and the ultra soft, scalar relativistic pseudo potentials[31] were used. The plane-wave kinetic energy cutoff for the wave-functions and the electronic density were chosen as 30 Ry and 240 Ry, respectively. The reaction energy was obtained as the difference between the total DFT energies of the product and those of the reactants during the reaction.
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ACCEPTED MANUSCRIPT 4. Conclusions In conclusion, a novel tridentate NTTTA ligand has been prepared and used as the stabilizer for AgNPs. The NTTTA-AgNPs assay exhibits a highly selectivity and sensitivity for colorimetric sensing of Mn2+ in aqueous solution. The effect of the concentration of NTTTA ligand on the selectivity and sensitivity of the sensor has been investigated. Under the optimized condition, the
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NTTTA-AgNPs assay illustrated good selectivity for Mn2+ detection over 15 other metal ions and 8 anions. The detection limit of Mn2+ was approximately 0.5 µM by the naked eyes and 12.6 nM (S/N
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= 3) by the UV-vis spectroscopy analysis, respectively. The 1,2,3-triazole branches of NTTTA ligand played important role in coordinating with Mn2+ through bi-dentate coordination, which cause cross-
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linking and aggregation of AgNPs. The detection process was simple without the need of adjusting the pH value. In addition, the current assay could be applied to the detection of Mn2+ in tap water
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and lake water samples.
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Acknowledgements
We are very grateful to Dr. Jinguang Cai for Raman spectra discussion. Financial
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support from the Natural Science Foundation of China (21507117, 21601166), the C hina
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Academy of Engineering Physics (PY201409, PY201410) and Discipline Development Foundation of Science and Technology on Surface Physics and Chemistry Laboratory
References
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(XKFZ201505) are gratefully acknowledged.
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ACCEPTED MANUSCRIPT based tools and strategies for heavy-metal detection, Chem. Rev. 111 (2011) 3433-3458. [6] E. Oliveira, C. Núñez, H. M. Santos, J. Fernández-Lodeiro, A. Fernández-Lodeiro, J. L. Capelo, C. Lodeiro, Revisiting the use of gold and silver functionalised nanoparticles as colorimetric and fluorometric chemosensors for metal ions, Sens. Actuators B Chem. 212 (2015) 297-328.
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ACCEPTED MANUSCRIPT [17] L. Chen, Y. Ye, H. Tan, Y. Wang, A simple and rapid colorimetric method for the determination of Mn2+ based on pyrophosphate modified silver nanoparticles, Colloids and Surfaces A: Physicochem. Eng. Aspects 478 (2015) 1–6. [18] Y. Zhou, H. Zhao, C. Li, P. He, W. Peng, L. Yuan, L. Zeng, Y. He, Colorimetric detection of Mn2+ using silver nanoparticles cofunctionalized with 4-mercaptobenzoic acid
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ACCEPTED MANUSCRIPT [27] D. Huang, P. Zhao,D. Astruc, Catalysis by 1,2,3-triazole- and related transition-metal complexes, Coord. Chem. Rev. 272 (2014) 145-165. [28] P. Giannozzi, S. Baroni, N. Bonini, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli, G. L. Chiarotti, M. Cococcioni, I. Dabo, Quantum espresso: a modular and open-source software project for quantum simulations of materials, J. Phys.: Condens. Matter 21 (2009) 395502 (19
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Graphical Abstract
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ACCEPTED MANUSCRIPT Highlights A novel tridentate ligand nitrilotris(methylene)tris(1,2,3-triazole)triacetate (NTTTA) has been synthesized and used as modifier of silver nanoparticles (AgNPs). The NTTTA-AgNPs showed highly selective and sensitive colorimetric detection of Mn2+ in aqueous solution without pH control.
Mechanism studies suggested that the 1,2,3-triazole play important role in the selective of Mn2+.
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Jianyu Wei, Jinfan Chen, Guozong Yue, Liangsheng Hu, Danqing Zhao, Jing Zhu, Luming Yang, Deshun Huang, Pengxiang Zhao PII: DOI: Reference:
S1386-1425(18)30424-4 doi:10.1016/j.saa.2018.05.033 SAA 16067
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received date: Revised date: Accepted date:
8 February 2018 3 May 2018 8 May 2018
Please cite this article as: Jianyu Wei, Jinfan Chen, Guozong Yue, Liangsheng Hu, Danqing Zhao, Jing Zhu, Luming Yang, Deshun Huang, Pengxiang Zhao , Development of a novel tridentate ligand for colorimetric detection of Mn2+ based on AgNPs. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Saa(2017), doi:10.1016/j.saa.2018.05.033
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ACCEPTED MANUSCRIPT Development of a novel tridentate ligand for colorimetric detection of Mn2+ based on AgNPs Jianyu Weia,b, Jinfan Chena,Guozong Yuea, Liangsheng Huc, Danqing Zhaoa, Jing Zhua, Luming Yangb, Deshun Huanga* and Pengxiang Zhaoa* a
Institute of Materials, China Academy of Engineering Physics, No. 9, Huafengxincun, Jiangyou City, Sichuan
b
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Province, 621908, P. R. China. Email: [email protected]; [email protected] The Key Laboratory of Leather Chemistry and Engineering of Ministry of Education, Sichuan University,
c
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Chengdu, 610065, P. R. China.
Department of Chemistry and Key Laboratory for Preparation and Application of Ordered Structural Materials of
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Guangdong Province, Shantou University, Guangdong, 515063, P. R. China
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Abstract
A novel tridentate ligand nitrilotris(methylene)tris(1,2,3-triazole)triacetate (NTTTA) has been
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synthesized by click reaction and followed with ester hydrolysis reaction. The silver nanoparticles (AgNPs) were then modified and stabilized by this ligand, and subsequently been employed for the
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highly selective and sensitive colorimetric detection of Mn2+ in aqueous solution. The presence of
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Mn2+ can cause the aggregation of AgNPs, which leads to the color change of the dispersion from yellow to brown, as well as the decrease and red-shift of the surface plasmon resonance absorption. The detection limit of Mn2+ was as approximately 0.5 µM by the naked eyes. UV-vis spectroscopy
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analysis showed a good linear relationship between the logarithm of the ratios (A550/A395) and the concentration of Mn2+over the range of 0.05 µM-10 µM, and the LOD was calculated to be 12.6
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nM (S/N = 3). The present assay showed good simplicity without the need of adjusting the pH value. The feasibility of this technique was evaluated for successful detection of Mn2+ in tap water and lake water samples, with good recoveries.
Keywords: tridentate ligand; AgNPs; colorimetric detection; Mn2+
1. Introduction Colorimetric sensing based on gold nanoparticles (AuNPs) and silver nanoparticles (AgNPs) has attracted remarkable attention because of its simplicity, rapidity, and high sensitivity and selectivity [1]. AuNPs and AgNPs are excellent candidates for the fabrication of novel chemical and 1
ACCEPTED MANUSCRIPT biological sensors because of the unique physical and chemical properties [2,3]. Firstly, they possess high surface-to-volume ratio with ease of modification through suitable surface chemistry. Secondly, their localized surface plasmon resonances (LSPR) related optical properties can be readily tuned by varying their size, shape, and the surrounding chemical environment. Finally, their molar extinction coefficients were much greater compared to that of common organic dyes, which may
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increase the detection sensitivity when used in absorption spectroscopy [4]. It is noting that the molar extinction coefficient of AgNPs is ~100 fold greater than that of
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AuNPs. Therefore, as a sensor, AgNPs somehow should have a big advantage compared with AuNPs. Therefore, many colorimetric methods have been reported for detection of heavy metal ions
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based on gold or silver nanoparticles[4-6]. For an example, manganese ion (Mn2+) is one of the essential microelements in human body, which plays important roles in life processes[7]. However,
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excessive intake of Mn2+ is harmful, which may cause functional handicaps and some disorders to the endocrine, nervous and reproductive systems[8]. Therefore, much attention has been attracted
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to develop methods for detecting Mn2+ with high sensitivity and selectivity. Apart from conventional Mn2+ detection methods such as atomic absorption spectrometry (AAS) [9,10], inductively coupled
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plasma mass spectroscopy (ICP-MS) [11], and inductively coupled plasma-atomic emission
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spectrometry (ICP-AES)[12], colorimetric assay based on AuNPs and AgNPs has emerged as an more economical method. Colorimetric detections of Mn2+ based on AuNPs were reported using polyacrylamide (PAM) [13] or L-dopa[14] as modifiers with the detection limit of 5 µM. While
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those based on AgNPs were reported using clove (S. aromaticum) [15], L-arginine[16], P2O74-[17], 4-mercaptobenzoic acid (4-MBA) and melamine (MA) (4-MBA-MA) [18], P3O105-[19], β-
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cyclodextrin-adamantane (β-CD-ADM) [20], Na4P2O7 and hydroxypropylmethylcellulose (Na4P2O7-HPMC) [21] or cysteic acid (CA)[22] as modifiers, with the lowest detection limit of 20 nM. Although these methods have led to successful development of Mn2+sensors, they always need to adjust pH values which reduced the simplicity, and some of the detection limit is above the recommended limit of the US Environmental Protection Agency for drinking water (0.9 µM). Recently, we have reported a highly efficient way to detect the Cr(III) in aqueous solution by nitrilotriacetate (NTA) modified AuNPs[23]. It is disclosed that Cr(III) selectively coordinated with NTA on the AuNPs surface that led the aggregation of AuNPs. As well known, the surface modifiers have great impact on the sensitivity and selectivity of AuNPs or AgNPs.We suppose that some 2
ACCEPTED MANUSCRIPT modification of the NTA ligand could change the selectivity from Cr(III) to other metal ions.In this report, an analogue of NTA, a novel tridentate ligand nitrilotris(methylene)tris(1,2,3triazole)triacetate (NTTTA) has been synthesized through “click” formation of 1,2,3-triazole and the following ester hydrolysis reaction. The as prepared ligand has been used as a modifier of AgNPs to form the NTTTA-AgNPs assay, which showed highly sensitive and selective colorimetric
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detection of Mn2+ in aqueous solution, without the need of adjusting the pH value. Mechanism studies suggested that the 1,2,3-triazole play important role in changing the selectivity from Cr(III)
2. Results and discussion 2.1 The preparation and characterization of NTTTA-AgNPs
synthetic
procedure
of
the
tridentate
ligand
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The
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to Mn2+, through 1,2,3-triazole-carboxy-Mn coordination.
nitrilotris(methylene)tris(1,2,3-
triazole)triacetate (NTTTA) stabilized AgNPs was shown in Scheme 1. Firstly, tripropargylamine
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(1) and ethyl 2-azidoacetate (2) were synthesized by common nucleophilic substitution reactions respectively. Then 1 and 2 underwent the Cu-catalyzed alkyne-azide cycloaddition (CuAAC) (“click”
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reaction) [24] to form the triazole compound 3. The compound 3 was then hydrolyzed in NaOH
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solution and subsequently acidified to obtain the NTTTA-acid, compound 4. The obtained NTTTAacid was characterized using nuclear magnetic resonance spectrum (NMR) (see supporting information), high resolution mass spectrometry (HRMS). Finally, the target NTTTA , compound 5
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was generated by basification of the NTTTA-acid using three equivalents of NaOH. In order to the prepare the NTTTA-AgNPs assay, unmodified AgNPs were firstly synthesized using NaBH4, and
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subsequently capped with the NTTTA ligand (Scheme 1).
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Scheme 1 The preparation of NTTTA stabilized AgNPs.
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The obtained NTTTA-AgNPs appeared a pale yellow color which was stored in a refrigerator
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at 4 oC before use. UV-vis. spectroscopy indicated a maximum absorption wavelength at 395 nm, which was slightly red-shifted compared with the unmodified AgNPs (390 nm) (Fig. S1†). The
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TEM images indicated that the NTTTA-AgNPs were well dispersed in water with an average
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particle size of 11 nm (Fig. 1A and S2†). However, after mixing with 10 µM Mn2+ ions, aggregation
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of NTTTA-AgNPs happened which could be shown in the TEM images (Fig 1B).
Fig. 1 TEM images of NTTTA-AgNPs before (A) and after (B) mixing with Mn2+ (10 µM).
2.2 Optimization of experimental conditions
In order to investigate the effect of the NTTTA concentration on the selectivity and sensitivity of the sensor, several NTTTA-AgNPs with different molar ratios of NTTTA ligand and AgNO3
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ACCEPTED MANUSCRIPT (NTTTA/AgNO3 = 0.25, 0.5, 1, and 2) were prepared and subjected to the colorimetric assay. Fig. 2A showed the relationship between A550/A395 ratios and NTTTA/AgNO3 ratios, where A550/A395 represents the absorbance ratios between the aggregated (A550) and non-aggregated (A395) AgNPs, and the higher ratio indicated the higher sensitivity or lower stability. The result revealed that, with the decrease of NTTTA/AgNO3 ratios, the stability of AgNPs slightly decreased while the sensitivity
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of the NTTTA-AgNPs assay for the Mn2+ detection increased greatly. However, if the NTTTA/AgNO3 ratios were too small (0.25 and 0.5), some other metal ions like Cu2+ and Cd2+
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sensitivity, the NTTTA/AgNO3 ratio was optimized to be 1.
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interfered the Mn2+ detection (Fig. 2B). Consequently, on the consideration of both selectivity and
Fig. 2 (A) The effect of NTTTA/AgNO3 ratios on the stability and sensitivity of NTTTA-AgNPs.
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(B) Photographic images of NTTTA-AgNPs in the presence of Mn2+ (10 µM) or other metal ions (30 µM), with NTTTA/AgNO3 ratios of 0.25 (I), 0.5 (II), 1 (III), and 2 (IV).
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Subsequently, the incubation time of the probe was also tested for the colorimetric detection of Mn2+. In the presence of 10 µM Mn2+, the A550/A395 ratio increased rapidly within 6 minute (Fig. S3 †). Further increase of time led to slow and little increase of the ratio. Therefore, 6 minute was selected as the optimized incubation time for further detection.
2.3 Selectivity for Mn2+ detection with NTTTA-AgNPs
In order to demonstrate the selectivity for Mn2+ detection with NTTTA-AgNPs system, eye vision and UV-vis spectra of the assay after adding of Mn2+ (10 µM) or other ions including Fe3+, Fe2+, K+, Al3+, Cu2+, Cd2+, Co2+, Ni2+, Pb2+, Zn2+, Hg2+, Mg2+, Ca2+, Cr3+, Cr2O72-, NO3-, PO43-, CO32-, 5
ACCEPTED MANUSCRIPT SO42-, Br-, NO2-, SCN-, and HCOO- (30 µM respectively) were investigated. As shown in Fig. 3, only the presence of Mn2+ (10 µM) led to an apparent decrease and red-shift of extinction band (Fig. 3A), as well as the color change from pale-yellow to brown (Fig. 3B), whereas the other types of ions had slight changes even at much higher concentration (3 times of Mn2+). Moreover, the A550/A395 ratio of NTTTA-AgNPs after mixing with Mn2+ was considerably larger than the ratios of
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which mixing with other ions (Fig. 3B).
Fig. 3 (A) UV-vis spectroscopy of NTTTA-AgNPs and (B) A550/A395 ratios and photographic image (inset) of NTTTA-AgNPs after the addition of Mn2+ (10 µM) and other ions (30 µM respectively).
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The influence of the presence of other ions towards the detection of Mn2+ was also investigated.
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It is obvious the color and A550/A395 ratios of the NTTTA-AgNPs incubated with the mixture of Mn2+ and other ions (Fig. 4) were all similar with those only incubated with Mn2+ (Fig. 3B). These results indicated that the presence of other ions had little interference on the Mn2+ detection because
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they cannot inhibit the aggregation of the AgNPs induced by Mn2+.
Fig. 4 A550/A395 ratios and photographic images (inset) of NTTTA-AgNPs incubated with Mn2+ (10 µM) and another ions (10 µM, respectively). 6
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2.4 Sensitivity forMn2+ detection with NTTTA-AgNPs
The sensitivity of the NTTTA-AgNPs assay towards Mn2+ detection was conducted by eye vision and UV-vis spectra under optimized conditions. As shown in Fig. 5, with the increase of the Mn2+ concentration (from 0.05µM to 20 µM), the color of the NTTTA-AgNPs changed gradually
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from pale-yellow to brown. The detection limit of Mn2+ was approximately 0.5 µM by eye vision. The UV-vis spectra of AgNPs shown in Fig. 5A indicated that, with the increase of Mn2+
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concentration, the LSPR absorption at 395 nm gradually decreased, while the red-shifted absorption band at 550 nm gradually increased. Fig. 5B showed the relationship between logarithm of the
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A550/A395 ratios of different AgNPs dispersions and concentrations of Mn2+ ranging from 0.05 to 20 µM. As shown in the inset of Fig. 5B, a good linear relationship (Y=-1.20183+0.05377X) was
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obtained over the range of 0.05 μM-10 μM with the correlation coefficient of 0.99283, and the LOD was 12.6 nM (S/N = 3) by UV-vis spectroscopy. Comparing with the reported colorimetric sensors,
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the current reported NTTTA-AgNPs system showed simple and high sensitivity without pH control
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(Table 1).
Fig. 5(A) UV-vis spectroscopy and photographic images (inset) of NTTTA-AgNPs after mixing with different concentration of Mn2+ (0.05-20 µM). (B) Plot of lg(A550/A395) as a function of the concentrations of Mn2+ and the linear calibration plot for the concentrations of Mn2+ from 0.05 to 10 µM (inset). Table 1. Comparison of various colorimetric sensors for the detection of Mn2+. Detection system
pH
LOD
Incubation time
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ref
-
5 µM (eye)
10 min
[14]
PAM-AuNPs
4.5
5 µM (S/N = 3)
2 min
[13]
AgNPs-L-arginine
9.4
20 nM (S/N = 3)
40 min
[16]
P2O74--AgNPs
~10
0.03 µM (S/N = 3)
20 min
[17]
4-MBA-MA-AgNPs
8
50 nM (S/N = 3)
5 min
[18]
clove-AgNPs
11.5
0.2 µM (eye)
30 min
[15]
P3O105−-AgNPs
~10.8
0.1 µM (eye)
Several min
[19]
β-CD-ADM AgNPs
-
0.5 µM (UV-vis)
5 min
[20]
Na4P2O7-HPMC-AgNPs
12
0.5 µM (eye)
10 min
[21]
CA-AgNPs
7.9
5 nM (UV-vis)
30 s
[22]
NTTTA-AgNPs
-
0.5 µM (eye)
6 min
This work
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L-dopa-AuNPs
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12.6 nM (S/N = 3)
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2.5 Detection of Mn2+ in real water samples with the NTTTA-AgNPs assay
The feasibility of the current NTTTA-AgNPs assay has been evaluated by subjecting the real
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water samples (tap water and lake water), which were spiked with various concentrations of Mn2+
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standard solution, to the detection system. The results disclosed that the concentrations determined with the current method (third column in Table 2) were very close to the real spkied concentrations of the samples (second column in Table 2). Therefore, the NTTTA-AgNPs assay could be realistic
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for the determination of Mn2+ in tap water and lake water samples. Table 2. The results of the detection of Mn2+ in real water samples with the NTTTA-AgNPs assay.
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Samples
Tap water
Lake water
Spiked (μM)
Found (n=3) (μM)
Recovery (%)
RSD/%
0.50
0.544
108.8
7.97
2.0 6.5
1.981 6.466
99.05 99.48
0.91 0.21
1.0
1.016
101.6
1.31
3.0
3.034
101.13
1.2
7.0
7.042
100.6
0.42
2.6 The mechanism of Mn2+ detection
Basically, the as-prepared NTTTA-AgNPs were modified and stabilized by the electrostatic
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ACCEPTED MANUSCRIPT repulsion between the negatively charged NTTTA ions and the coordination between the tridentate NTTTA ligands and the AgNPs surface. Form the UV-vis. spectroscopy and TEM images (Fig. S1 † and 1), it can be concluded that the colorimetric detection of Mn2+ was based on the aggregation of NTTTA-AgNPs. We further studied the aggregation mechanism using FT-IR spectra and Raman spectra analysis.
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FT-IR spectra of NTTTA, NTTTA-AgNPs in the absence and presence of Mn2+ were firstly characterized and shown in Fig. 6. The FT-IR spectra of NTTTA showed strong peaks at 1617 cm-1
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and 1391 cm-1 which represented the carboxylate groups. The FT-IR spectra of NTTTA-AgNPs showed similar peaks at 1627 cm-1 and 1384 cm-1 which indicated the successful modification of
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NTTTA on the AgNPs surface. Then, in the presence of Mn2+, the peak at 1627 cm-1 shifted to 1631
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cm-1, which suggested coordination between the carboxylate group of NTTTA and Mn2+.
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Fig. 6 FT-IR spectra of NTTTA, NTTTA-AgNPs, and NTTTA-AgNPs in the presence of Mn2+. Subsequently, Raman spectra of NTTTA, NTTTA-AgNPs, NTTTA-AgNPs in the presence of
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Mn2+ with different concentrations were also recorded. As shown in Fig. 7, the Raman signal of NTTTA alone could not be detected. However, the NTTTA-AgNPs exhibited strong surfaceenhanced Raman scattering (SERS) effect and yielded strong vibrational signals of the NTTTA ligand, which further indicated the successful modification of NTTTA on the AgNPs surface[25]. In addition , the Raman peak at 1561 cm-1 could be contributed to the N=N stretching of the 1,2,3triazole group. It could be seen from Fig. 7, this peak was largely enhanced in the presence of increasing concentration of Mn2+, accompanied with slight blue-shift. These changes could be attributed to binding of Mn2+[26], which suggested the coordination between the 1,2,3-triazole group of NTTTA and Mn2+.
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Fig. 7 Raman spectra of NTTTA, NTTTA-AgNPs, and NTTTA-AgNPs in the presence of Mn2+(0.5 μM and 6 μM).
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As we known, metal ions are prone to coordinate with the 1,2,3-triazole to form five or sixmembered cycles, through bi-dentate coordination with both the donor site adjacent to N1 and the
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N2 site on the 1,2,3-triazole [27]. Based on the FT-IR spectra and Raman spectra analysis, it could be concluded that in the presence of Mn2+, the Mn2+ could be preferred to form inter-molecular
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coordination with two branches of NTTTA ligands on the AgNPs surface, through bi-dentate
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coordination with the carboxyl oxygen atom adjacent to N1 and the N2 atom on the 1,2,3-triazole,
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which cause cross-linking of AgNPs and leading the aggregation (Scheme 2).
Scheme 2 Possible mechanism of the NTTTA-AuNPs based colorimetric detection of Mn 2+. We then performed density functional theory (DFT) calculations to further explain the coordination mode between NTTTA and Mn2+. The change of the energy (ΔE) of the coordination
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ACCEPTED MANUSCRIPT interactions between Mn2+ and NTTTA ligand based on the above coordination mode can be calculated by the following formula 34Mn(H2O)62 2NTTTA Mn(NTTTA) 2(H2O)2 4H2O
(1)
That is, the hexahydrated Mn2+ ion underwent ligand exchange with two NTTTA molecules, forming a hexacoordinate structure with the Mn2+ ion serving as the coordination core while the
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carboxyl oxygen atom and the N2 atom on the 1,2,3-triazole from two NTTTA molecules and two water molecules coordinate to the core (Fig. 8). Based on optimized configuration, the ΔE was
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calculated to be -305.9 kcal/mol, which indicates that the proposed coordination mode is very
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energetically favorable.
3. Experimental
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Fig. 8 The calculated coordination model between Mn2+ and NTTTA ligand.
3.1 Materials and instrumentation
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All commercially available reagents were used without any further purification. K2Cr2O7 was used to prepare the Cr2O72- standard solution. PbNO3 was used to prepare the Pb2+ standard solution.
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Chloride salts of other metal salts were used to prepare the standard solution of other metal ions. NaNO3, Na3PO4, Na2CO3, Na2SO4, KBr, NaNO2, KSCN, and HCOONa were used to prepare the standard solution of respective anions. Glassware was washed with aqua-regia (HCl:HNO3=3:1). Ultra-pure water was used in all the experiments unless otherwise stated. UV-vis spectroscopy measurements were carried out by a UV-1800 SHIMADZU spectrometer. FT-IR spectra were obtained by a Nicolet-iS10 instrument. 1H and 13C NMR spectroscopy measurements were carried out by using a Bruker Avance 300 Spectrometer. HRMS spectra measurements were obtained through an AxION 2 TOF Mass Spectrometer. The TEM images were acquired by a STEM (JEOL JEM-2100F) instrument operated at 200 kV. Raman spectra were obtained on a Renishaw 2000 11
ACCEPTED MANUSCRIPT instrument. 3.2 The preparation of NTTTA NTTTA was synthesized by Cu-catalyzed alkyne-azide (CuAAC) reaction using tripropargylamine (1) and ethyl 2-azidoacetate (2), and followed by hydrolysis reaction in the presence of NaOH. Firstly, tripropargylamine (1) and ethyl 2-azidoacetate (2) were prepared using
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the following procedure. To the stirred solution of CH2Cl2 (50 mL) were added 3-bromopropyne (6.248 g, 42 mmol, 80%
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wt in toluene), 2-propynylamine (1.1016 g, 20 mmol) and NaOH (2 g, 50 mmol). After stirring at
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room temperature for 5 days, the resulting suspension was diluted with water (20 mL) and extracted by CH2Cl2 (2×20 mL).The organic layer was washed with water (20 mL), dried over anhydrous
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Na2SO4, filtered, and concentrated under reduced pressure to get the compound 1. 1H NMR (300 MHz, CDCl3) δ 3.48 (d, J = 2.4 Hz, 6H), 2.26 (t, J = 2.4 Hz, 3H).
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Ethyl bromoacetate (3.34 g, 20 mmol) and sodium azide (1.56 g, 24 mmol) were added into a mixture solution of H2O (10 mL) and acetone (30 mL). After stirring at room temperature for 5h,
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the resulting solution was diluted with water (20 mL) and extracted by CH2Cl2 (2×20 mL). The
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organic layer was washed with water (20 mL), dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure to get the compound 2. 1H NMR (300 MHz, CDCl3) δ 4.27 (q, J = 6.0 Hz, 2H), 3.86 (s, 2H), 1.31 (t, J = 6.0 Hz, 3H).
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Accordingly, to the stirred mixture of compound 1 (3.87 g, 30 mmol) and 2 (1.31 g, 10 mmol) in THF (80 mL) were added the aqueous solution of CuSO4 (30 mL, 0.17 M) and sodium ascorbate
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(30 mL, 0.3 M) at room temperature under nitrogen atmosphere. The resulting solution was stirred overnight at room temperature. After removal of THF in vacuo, CH2Cl2 (30 mL) and aqueous EDTA (30 mL, 0.2 M) were added to the reaction mixture, which was stirred for 30 min to remove the Cu ions. The organic layer was separated, and the water phased was extract with CH2Cl2 (30 mL). The organic layer was collected, washed with water (30 mL), dried over anhydrous Na2SO4, filtered, and concentrated under reduced pressure to get the crude compound 3, which was subjected to the hydrolysis reaction directly. To the stirred solution of compound 3 in methanol (30 mL) was added NaOH (1.22 g, 30.5 mmol) under room temperature. After stirring for 3h, aqueous HCl (1 M) was added to adjust the pH value to approximately 3.0. The resulting mixture was concentrated under 12
ACCEPTED MANUSCRIPT reduced pressure to the obtained the crude product which was purified by silica column chromatography (EtOAc/MeOH, 1:1 to 1:5) to obtain the target NTTTA-acid (4) (acid form) as pale yellow solid. 1H NMR (300 MHz, D2O) δ 8.18 (s, 3H), 5.06 (s, 6H), 4.49 (s, 6H); 13C NMR (75.4 MHz, D2O) 172.95, 136.79, 128.67, 55.18, 46.46. HRMS (ESI) Calcd for C15H17N10O6 (M-H): 433.1338; Found: 433.1348.
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3.3 The preparation of NTTTA-AgNPs NTTTA (5) (0.048 M) was firstly prepared by mixing the above synthesized NTTTA-acid with
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three equivalents of NaOH in aqueous solution. AgNPs were prepared by reducing AgNO3 with NaBH4. For details, NaBH4 (5 mg, 0.132 mmol) was rapidly added into the aqueous solution of
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AgNO3 (50 mL, 1×10-4 M) under vigorously stirring at room temperature.The resulting solution was stirred for 10 min, during which time the color changed from colorless to pale-yellow. Subsequently,
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aqueous solution of NTTTA (0.104 mL, 0.048 M) was added quickly into the above solution with continuous stirring for another 30 min. The obtained NTTTA-AgNPs was stored in a refrigerator at
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4oC and subjected to the detection process directly without further purification. 3.4 Colorimetric Detection of Mn2+ using NTTTA-AgNPs
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Mn2+ aqueous solution (0.1 mL) of certain concentrations was homogeneous mixed with the
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as prepared NTTTA-AgNPs (0.9 mL), which was incubated at room temperature for 6 min.Then the photographic images and UV-vis spectroscopy measurements were recorded. 3.5 Computational details
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Spin-polarized DFT simulations were carried out using the PWscf module implemented in the plane-wave Quantum_Espresso package[28] to study the coordination reactions. The molecular
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cluster was situated in a cubic unit cell with the side-length of 35 Å. Gamma-point sampling was adopted and the Makov-Payne method[29] was used to obtain the correction to the total energy due to the use of the periodic conditions in the calculations. The generalized gradient approximations (GGA) Perdew-Burke-Ernzerhof (PBE)[30] functional, and the ultra soft, scalar relativistic pseudo potentials[31] were used. The plane-wave kinetic energy cutoff for the wave-functions and the electronic density were chosen as 30 Ry and 240 Ry, respectively. The reaction energy was obtained as the difference between the total DFT energies of the product and those of the reactants during the reaction.
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ACCEPTED MANUSCRIPT 4. Conclusions In conclusion, a novel tridentate NTTTA ligand has been prepared and used as the stabilizer for AgNPs. The NTTTA-AgNPs assay exhibits a highly selectivity and sensitivity for colorimetric sensing of Mn2+ in aqueous solution. The effect of the concentration of NTTTA ligand on the selectivity and sensitivity of the sensor has been investigated. Under the optimized condition, the
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NTTTA-AgNPs assay illustrated good selectivity for Mn2+ detection over 15 other metal ions and 8 anions. The detection limit of Mn2+ was approximately 0.5 µM by the naked eyes and 12.6 nM (S/N
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= 3) by the UV-vis spectroscopy analysis, respectively. The 1,2,3-triazole branches of NTTTA ligand played important role in coordinating with Mn2+ through bi-dentate coordination, which cause cross-
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linking and aggregation of AgNPs. The detection process was simple without the need of adjusting the pH value. In addition, the current assay could be applied to the detection of Mn2+ in tap water
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and lake water samples.
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Acknowledgements
We are very grateful to Dr. Jinguang Cai for Raman spectra discussion. Financial
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support from the Natural Science Foundation of China (21507117, 21601166), the C hina
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Academy of Engineering Physics (PY201409, PY201410) and Discipline Development Foundation of Science and Technology on Surface Physics and Chemistry Laboratory
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(XKFZ201505) are gratefully acknowledged.
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Graphical Abstract
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ACCEPTED MANUSCRIPT Highlights A novel tridentate ligand nitrilotris(methylene)tris(1,2,3-triazole)triacetate (NTTTA) has been synthesized and used as modifier of silver nanoparticles (AgNPs). The NTTTA-AgNPs showed highly selective and sensitive colorimetric detection of Mn2+ in aqueous solution without pH control.
Mechanism studies suggested that the 1,2,3-triazole play important role in the selective of Mn2+.
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