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Highly Selective and Sensitive Detection of Pb2+ in Aqueous Solution Using Tetra(4-pyridyl)porphyrin Functionalized Thermo-Sensitive Ionic Microgels Bin Wen, Jinqiao Xue, Xianjing Zhou, Qingwen Wu, Jingjing Nie, Junting Xu, and Binyang Du ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08497 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018
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ACS Applied Materials & Interfaces
Highly Selective and Sensitive Detection of Pb2+ in Aqueous Solution Using Tetra(4-pyridyl)porphyrin Functionalized Thermo-Sensitive Ionic Microgels Bin Wen, † Jinqiao Xue, † Xianjing Zhou,ξ Qingwen Wu, † Jingjing Nie, ‡ Junting Xu† and Binyang Du †* †
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China ξ
Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, China ‡
Department of Chemistry, Zhejiang University, Hangzhou 310027, China
Keywords. Tetra(4-pyridyl) porphyrin, thermosensitive ionic microgels, Pb2+ ion detection, nanomolar level.
ABSTRACT. Tetra(4-pyridyl)
porphyrin
(TPyP)
functionalized
thermosensitive
ionic
microgels
(TPyP5-MGs) were synthesized by a two-step quaternization method. The obtained TPyP5-MGs have hydrodynamic radius of about 189 nm with uniform size distribution and exhibit thermosensitive character. The TPyP5-MG microgel suspensions can optically response to trace Pb2+ ions in aqueous solution with high sensitivity and selectivity over the interference of other 19 species of metal ions (Yb3+, Gd3+, Ce3+, La3+, Bi3+, Ba2+, Zn2+, Ni2+, Co2+, Mn2+, Cr3+, K+, Na+, Li+, Al3+, Cu2+, Ag+, Cd2+, and Fe3+) by using UV-visible spectroscopy. The sensitivity of TPyP5-MGs toward Pb2+ can be further improved by increasing the solution temperature. The limit of detection for TPyP5-MG microgel suspensions in the detection of Pb2+ in aqueous solution at 50 oC is about 25.2 nM, which can be further improved to be 5.9 nM by using the method of higher-order derivative spectrophotometry (HODS) and is much lower than the U. S. EPA standard for the safety
*
Corresponding author. E-mail:
[email protected].
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limit of Pb2+ ions in drinking water. It is further demonstrated that the TPyP5-MG microgel suspensions have potential application in the detection of Pb2+ in real world samples, which give consistent results with those obtained by elemental analysis.
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INTRODUCTION Because of indecent emission without formal post-processing, the pollution of heavy metal ions in aqueous solutions is causing serious threats to the issues of human health and ecological environment. What is even more besorgniserregend is that this hazard has been involved in various aspects like groundwater, daily necessities and industrial supplies and so forth. Among heavy metal ions, lead ion (Pb2+), which is widely existing in such areas as batteries, agricultural wastewater, non-ferrous metal smelting industry and motor vehicle emissions, is one of the most toxic heavy metal ions and generally considered as a potential possibility to cause adverse effects on metabolism, intelligence and some diseases when its concentration exceeds or accumulates the maximum permissible limit.1 Due to the toxicity of Pb2+, the maximum allowable level of Pb2+ in drinking water cannot exceed 15 µg/L (or 72 nM) in the guideline of the Environmental Protection Agency (EPA) of United States. On the other hand, the World Health Organization has also established that the maximum allowable level of Pb2+ in drinking water cannot exceed 10 µg/L (or 48 nM) in its guideline for drinking water. Therefore, many analytical strategies have been extensively investigated and developed for detection of trace Pb2+ in aqueous solution, like flame atomic absorption spectrometry (FAAS), electrothermal atomic absorption spectrometry (ETAAS), graphite furnace atomic absorption spectrometry (GFAAS), inductively coupled plasma atomic emission spectrometry (ICP-AES), and inductively coupled plasma mass spectrometry (ICP-MS), colorimetric method, electrochemical method, atomic fluorescence spectrometry, differential pulse anodic stripping voltammetry (DPASV) and X-ray fluorescence spectrometry.2-10 For example, Bagheri et al.11 synthesized nanostructure Pb2+ ion-imprinted polymer by using diphenyl carbazone as the ligand for the extraction of Pb2+ ions from various matrixes. This method overcame the lack of co-extraction with the interfering compounds and target analytes, and the limit of detection of the method determined by FAAS was 0.42 ng/mL.11 Behbahani et al.
12
combined
solvent assisted dispersive solid phase extraction (SA-DSPE) and FAAS by using dithizone as a suitable chelation agent and reached a detection limit of 1.2 µg/L for Pb2+ in fruit and water
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samples.
Ma and Lin et al.
13
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developed an electrochemical metal ions sensor, based on a phytic
acid functionalized polypyrrole/graphene oxide (PPy/GO) nanocomposites modified electrode, to analyze various trace metal ions by differential pulse voltammetry (DPV). They achieved a linear working range of 5–150 µg/L for the detection of Pb2+.13 Liu et al.10 reported that the monodispersed gold nanoparticles inside carbon foam frameworks (AuNPs/CFs) exhibited excellent electrochemical response in the detection of Pb2+ with limits of detection of 5.2 nM by using differential pulse anodic stripping voltammetry (DPASV). Guo at al. 14 used the reduced graphene oxide and Chitosan hybrid matrix/poly-l-lysine films (RGO-CS/PLL) modified glassy carbon electrode for simultaneous electrochemical determination of Pb2+ by using DPASV, of which the detection limit is 0.02
µg/L.
Awual
et
al.
15
used
mesoporous
silica,
anchored
by
N,
N’
di(3-carboxysalicylidene)-3,4diamino-5-hydroxypyrazole (DSDH), as nano-adsorbent for sensitive detection and selective removal of Pb2+ in aqueous solutions, of which the detection limit was 0.29 µg/L. Although there are many analytical methods that can be utilized to detect Pb2+ ions in aqueous solutions, most of them suffer some restrictions including complicate pre-treatment or expensive, highly cost or time consuming, and are not available for routine analysis in various environments. Therefore, it is worthy to develop simple, sensitive, selective, low cost yet robust methods for the detection of trace Pb2+ ions in aqueous solutions. It is well documented that porphyrin can form complex with various metal ions. The sensors based on porphyrin derivatives have been then constructed for detecting trace heavy metal ions in environments. For example, Gupta et al.
16
found that poly(vinyl chloride)-based membranes of
meso-tetrakis-{4-[tris-(4-allyl dimethylsilyl-phenyl)-silyl]-phenyl}porphyrin (I) and (sal)2 trien (II) as electroactive material with plasticising solvent mediators could be acted as Ni2+ selective sensor in the activity range of 2.5 × 10−6 to 1.0 × 10−1 M. Zhang et al.
17
developed a
naphthalimide-porphyrin hybrid based ratiometric bioimaging probe for ratiometric detection of Hg2+ over a concentration range from 1.0 × 10-7 to 5.0 × 10-5 M with a detection limit of 2.0 × 10-8 M in aqueous solution and living cells. Balaz et al. 18 reported a single-labeled pyridylporphyrin–
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DNA conjugate, zinc(II) pyridylporphyrin-5’-oligodeoxythymidine (ZnPorT8), as highly sensitive and selective spectroscopic sensors for Hg2+ in water with a detection limit of 21 nM. Lodeiro et al. 19
prepared
pyrazole−porphyrin
2-(1,3-Diphenyl-1H-pyrazol-5-yl)-5,10,15,20-tetraphenylporphyirin
conjugates, and
2-(1-Phenyl-3-(2-pyridil)-1H-pirazol-5-yl)-5,10,15,20-tetraphenylporphyrin for optical detection of Zn2+, Cd2+ and Hg2+ in aqueous solutions. Jiang et al. tetra(aryl)porphyrin
derivative,
20
synthesized a metal-free
5-[p-N,N-bis(2-pyridylmethyl)amino-phenyl]-10,15,20-tris
(4-tert-butylphenyl)porphyrin (porphyrin-1-DPA), which can be used as a ratiometric Pb2+ sensor with a detection limit of 3.1 × 10-7 M. Buntem et al.
21
prepared a solid metal ion sensor doping
meso-tetra(p-carboxyphenyl)porphyrin into a silica monolith for differentiation of Cu2+, Zn2+, Pb2+, and Ni2+ in aqueous solutions.
Although some porphyrin-based sensors have been developed for
the detection of Pb2+ in aqueous solutions, the selectivity and sensitivity of Pb2+ sensing still need to be improved. Furthermore, many porphyrin derivatives are hydrophobic and insoluble in aqueous solutions. Therefore, effective strategies are required to incorporate porphyrin moieties into a hydrophilic environment, which might significantly enhance the detection selectivity and sensitivity of porphyrin moieties toward targeted metal ions in aqueous solutions. In the present work, a porphyrin derivative, 5,10,15,20-tetra(4-pyridyl) porphyrin (TPyP), is successfully incorporated into the three-dimensional crosslinked network of thermo-sensitive microgels via quaternization method. The thermosensitive microgels can response to the external stimuli, like temperature, pH and ionic strength, etc.22-34 By introducing specific functionality into the microgel networks, the thermosensitive microgels have been explored as pH and temperature sensors,35-36 heavy metal ion sensors,37-39 biomolecule sensors,40-42 organophosphate sensors43 and the like. Herein, TPyP was first quaternized with excess 1,5-dibromopentane (Br-C5H10-Br) to give a precursor solution, TPyP5, which was then used as crosslinking agent for the preparation of microgels. TPyP functionalized thermosensitive ionic microgels (TPyP5-MGs) were synthesized via surface free emulsion polymerization (SFEP) by using N-isopropylacrylamide (NIPAm) as the
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main monomer and 1-vinylimidazole (VIM) as the quaternizable comonomer in the presence of TPyP5 precursor. The obtained TPyP5-MGs are spherical in shape with uniform size distribution and exhibit thermosensitive character. We shall show that the TPyP5-MG microgel suspensions can optically response to trace Pb2+ ions in aqueous solution with high sensitivity and selectivity over the interference of other 19 species of metal ions studied here by using UV-visible spectroscopy. Furthermore, increasing the solution temperature can improve the sensitivity of TPyP5-MGs toward Pb2+. The limit of detection for TPyP5-MG microgel suspensions in the detection of Pb2+ in aqueous solution is much lower than the safety limit of Pb2+ (∼ 72 nM) in drinking water permitted by the EPA standard of the United States. We also demonstrate that the TPyP5-MG microgel suspensions have potential application in the detection of Pb2+ in real samples, which give consistent results with those obtained by elemental analysis.
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ACS Applied Materials & Interfaces
Scheme 1. Synthesis Routine of TPyP Functionalized Thermosensitive Ionic Microgels Br
Br Br
N
N
N Br
N
N2 DMF 100 oC 48h
NH N
+
N
NH
TPyP5 precursor
Br
Br
N
N NH
NH
N
N Br
N Br
TPyP
N
Br
TPyP5
Br-C5H10-Br
Br
+ NH
+
N
N2
H2O 70 oC 6h
TPyP5 precursor
O
N
NIPAm
VIM TPyP5-MGs Microgels
x
y N
NH
N
O N Br N Br N
N
Br N
N N
Br
NH N
N NH
N Br N
Br N
N Br
N
Br
Br N N Br N
N
EXPERIMENTAL SECTION Materials. N-isopropylacrylamide (NIPAm, 99%) and 1,5-dibromopentane (Br-C5H10-Br, 98%) were supplied by Tokyo Chemical Industry Co. Ltd. 5,10,15,20-Tetra(4-pyridyl) porphyrin (TPyP, 97%) and 1-vinylimidazole (VIM, 99%) were purchased from J&K Chemical Ltd. 2,2’-Azobis(2-methylpropionamidine)
dihydrochloride
(AIBA,
97%)
was
obtained
from
Sigma-Aldrich. Lithium nitrate (LiNO3, 99%), cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O, 98%), bismuth(III) nitrate pentahydrate (Bi(NO3)3·5H2O, 98%), lanthanum(III) nitrate hydrate (La(NO3)3·H2O, 99%), cerium(III) nitrate hexahydrate (Ce(NO3)3·6H2O, 99%), gadolinium(III)
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nitrate hexahydrate (Gd(NO3)3·6H2O, 99.9%), ytterbium(III) nitrate pentahydrate (Yb(NO3)3·5H2O, 99.99%), manganese(II) nitrate tetrahydrate (Mn(NO3)2·4H2O, 97.5%) were purchased from J&K Chemical Ltd. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 99%), aluminum nitrate nonahydrate (Al(NO3)3·9H2O, 99%), iron(III) nitrate nonahydrate (Fe(NO3)3·9H2O, 98.5%), copper(II) nitrate trihydrate (Cu(NO3)2·3H2O, 99.5%), cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O, 99%), nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O, 98%), lead(II) dinitrate (Pb(NO3)2, 99%), barium nitrate (Ba(NO3)2, 99.5%), silver nitrate (AgNO3, 99.8%), sodium nitrate (NaNO3, 99%), potassium nitrate (KNO3, 99%), and N,N-dimethylformamide (DMF, 99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Chromium(III) nitrate nonahydrate (Cr(NO3)3·9H2O, 99%) was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Synthesis of TPyP Functionalized Thermosensitive Ionic Microgels. TPyP functionalized thermosensitive ionic microgels were prepared via a two-step procedure. In the first step, TPyP (12.37 mg, 0.02 mmol) was first reacted with Br-C5H10-Br (23 µL, 0.15 mmol) in 1 mL DMF solution at 100°C for 48 h under vigorous magnetic stirring in nitrogen atmosphere. The reacted mixture was then cooled down to give the cross-linker precursor, named as TPyP5 DMF solution. In the second step, the main monomer NIPAm (0.2264 g, 2 mmol) and quaternizable comonomer VIM (19 µL, 0.2 mmol) were added into deionized water (49 mL) in a 100 mL three-neck flask. The mixture was subsequently heated up to 70 °C and degassed for 30 min with N2 under vigorous magnetic stirring. The AIBA aqueous solution (1 mL, 25mg/mL) was then added to initiate the surfactant free emulsion polymerization (SFEP). After 10 min, the TPyP5 DMF solution was dropwise added into the reaction mixture. The copolymerization was continued at 70°C for 6 h. Afterward polymerization, the reaction mixture was cooled down to room temperature and then transferred into a dialysis tube with MWCO of 14000 and dialyzed in DMF and deionized water for 5 days, respectively. The DMF or deionized water was refreshed every 8 h. Finally, the purified microgels were collected and used for further experiments. The obtained TPyP functionalized thermo-sensitive microgels were named as TPyP5-MGs. The yield of TPyP5-MGs is about 54.3%.
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The thermo-sensitive ionic microgels without TPyP functionalization were also prepared via the copolymerization of NIPAm (0.2264 g, 2 mmol) and VIM (19 µL, 0.2 mmol) in the presence of Br-C5H10-Br (23 µL, 0.15 mmol) in 50 mL deionized water as described previously,39 which were then named as N-MGs. Characterization. The morphology of the obtained microgels was characterized by using a JEOL JEM-1200 transmission electron microscope (TEM) with 80 kV acceleration voltages. The TEM samples were prepared by casting the microgel suspension with an appropriate concentration onto Formvar-coated copper grids, followed to dry in air at room temperature before observation. Hydrodynamic radius Rh and size distribution of the microgels were characterized by dynamic light scattering (DLS) on a 90 Plus Particle Size Analyzer (Brookhaven Instruments Corp.) at scattering angle (θ) of 90°. UV-vis spectra were recorded on a Cary 100 instrument (Varian Australia Pty Ltd.). Fourier-transform infrared (FTIR) spectra were recorded on a Bruker TENSOR II. The photographs of microgels were taken by using a Canon EOS 70D digital camera. Elemental analysis was carried out by using a 730-ES instrument (Varian Pty Ltd.). 1H NMR spectrum was recorded on a Bruker Advance DMX 400 MHz spectrometer. The MALDI TOF mass spectrometry measurement was performed on a Bruker amaZon ETD mass spectrometer.
RESULTS AND DISCUSSION The preparation of precursor TPyP5 DMF solution is crucial for successfully incorporating TPyP moieties into the microgel network because of the hydrophobicity and steric hindrance of TPyP moieties. The attempt to directly introduce TPyP into the microgels via in-situ quaternization reaction during the SFEP of NIPAm and VIM in the presence of Br-C5H10-Br was failed. The post functionalization of N-MGs with TPyP was also failed. The main possible reason is ascribed to the fact that the quaternization of TPyP with bromo-compound requires elevated temperature and long reaction times in organic solvent. Although it is difficult to explicitly illustrate the detail structures of TPyP5 precursors, several possible structures might coexist because of the excess of
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Br-C5H10-Br, like partial quaternized TPyP with Br-C5H10-Br, fully quaternized TPyP, dimer, trimer or more TPyP connected via Br-C5H10-Br, as shown in Figure S1. The MS spectrum of TPyP5 (Figure S2a) confirmed the existence of possible structures of TPyP5 precursors, namely TPyP5-4, TPyP5-3, TPyP5-2’ or TPyP5-2’’, and TPyP5-1, as given in Figure S1. From the 1H NMR spectrum of TPyP5, chemical shifts of δx = 3.48 ppm, δy = 1.90 ppm, and δx = 1.62 ppm were observed for Br-C5H10-Br, which shifted to be δe = 3.19 ppm, δf = 2.47 ppm, δg = 2.23 ppm, δh = 2.06 ppm, δi = 0.92 ppm, respectively, after quaternzation reaction with TPyP as shown in Figure S2b. Nevertheless, it can be for sure that only those TPyP5 structures with bromo-functional groups could be incorporated into the microgels via quaternization reaction with VIM during SFEP. Nevertheless, TPyP moieties are indeed successfully incorporated into the microgels by this two-step procedure as described in experimental section. Figure 1a shows the TEM morphology of obtained TPyP5-MGs. The TPyP5-MGs are uniform spheres with average radius of about 158 ± 16 nm in dried state. The cross-linked networks of TPyP5-MGs are formed via the quaternization reaction between VIM and free Br-C5H10-Br or TPyP5 precursors with bromo-functional groups. FTIR spectrum of TPyP5-MGs confirms the existence of TPyP moieties in TPyP5-MGs. As compared to the FTIR spectrum of N-MGs, a distinct absorption band at 1593 cm-1, corresponding to the C=N axial deformation of the meso-attached pyridyl substituents in TPyP, is clearly observed for pure TPyP and TPyP5-MGs, as shown in Figure 1b. Other differences appear in the peaks at 800 cm-1 and 971 cm-1, which are assigned to the =C-H out-of-plane bending vibration and the N-H in-plane deformation vibration of TPyP, respectively.44
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(b)
TPyP
N-MGs
TPyP5-MGs
1800
1500
1200
900
600
-1
Wavenumber / cm
(C) 220
70 °C
40
Hydrodynamic Radius (nm)
140
100 80 60
0.6
0.36 0.34 0.32 0.30 0.28 20
30
40
50
60
70
Temperature (°C)
0.4
25°C 25°C
40
0.2
20 0
120
25 °C
70°C
A482 nm/A428 nm
0.8
0 100 120 140 160 180 200
160
0.40 0.38
60
20
180
(d) 1.0
80
Absorbance
200
100
Intensity
Heating Cooling
Intensity
Hydrodynamic Radius (nm)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
ACS Applied Materials & Interfaces
160 180 200 220 240
70°C
Hydrodynamic Radius (nm)
30
0.0
40 50 60 Temperature (°C)
70
200
300
400 500 600 Wavelength (nm)
700
800
Figure 1. (a) TEM image of TPyP5-MGs and (b) FTIR spectra of TPyP, N-MGs and TPyP5-MGs. (c) Hydrodynamic radius of TPyP5-MG microgel suspensions measured by DLS as a function of measuring temperature. Inset shows the size distribution of TPyP5-MG microgel suspensions at 25 °C and 70°C. (d) UV-visible absorption spectra of aqueous suspensions of TPyP5-MGs at various temperatures. Inset shows the corresponding A482nm/A428nm ratio as a function of measuring temperature.
The TPyP5-MGs exhibit thermo-sensitive character and the hydrodynamic radius Rh of TPyP5-MGs varies reversibly with solution temperature, as shown in Figure 1c. In a heating-cooling cycle from 25 to 70 oC, Rh of TPyP5-MGs almost linearly decreases with increasing the solution temperature upon heating from 25 to 60 oC and then reaches a plateau value above 60 o
C. Upon cooling, the effective radius Rh of TPyP5-MGs increases again along the similar track
without hysteresis. Rh of TPyP5-MGs at 25 °C is about 189 ± 4 nm with narrow size distribution (inset of Figure 1c), which is slightly larger than that obtained from the TEM images, i.e., 158 ± 16 ACS Paragon Plus Environment
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nm. At 70 °C where the poly(NIPAm-co-VIM) network segments collapse, Rh of TPyP5-MGs is about 145 ± 2 nm with narrow size distribution (inset of Figure 1c). UV-vis spectrum of TPyP5-MG microgel suspension exhibits a characteristic absorption peak at 428 nm and the absorption bands from 500 to 700 nm, which are assigned to the Soret band and Q band of porphyrin chromophore, respectively (Figure 1d).45 Interestingly, the UV-vis absorption of TPyP5-MG aqueous suspension is also thermo-sensitive. The intensity of absorption peak at 428 nm slightly increases with increasing the solution temperature from 25 °C to 70 °C, whereas the intensity of shoulder adsorption peak at 482 nm decreases. As a result, the A482nm/A428nm ratio decreases with increasing the solution temperature (inset of Figure 1d). The increase of absorption intensity at 428 nm may be attributed to the collapse of TPyP5-MGs when raising the solution temperature, resulting in the less transmission of light. On the other hand, the decrease of absorption peak at 482 nm is mainly caused by the breaking of hydrogen bonds between o-hydroxyl proton and pyrrole group in TPyP moieties at a higher temperature and the increase of electron density of the conjugated system.39 The results of Figure 1d further indicate the successful incorporation of TPyP moieties in TPyP5-MGs. By referring to the standard calibration curve of TPyP in DMF (Figure S3), the contents of TPyP moieties in TPyP5-MGs are determined to be about 1.87 wt.% based on the concentration of TPyP5-MGs (0.15 mg/mL) and the intensity (0.596 a. u.) of absorption peak at 428 nm at 25 °C. Note that TPyP DMF solution also exhibits the characteristic absorption peak at 428 nm (Figure S3). Because of the chemical versatility and well-defined molecular structure of porphyrin, it has been widely used to form complex with various metal ions. By reasonable design and fabrication, various functional porphyrin-based sensors have been developed for the detection of heavy metal ions in aqueous solutions.46-48 To test the reaction activity of TPyP5-MGs with metal ions, 20 species metal ions with concentration of 100 µM are mixed with TPyP5-MG microgel suspensions at 25 °C, respectively. Note that the concentration of TPyP5-MG microgel suspensions is 0.15 mg/mL. It can be clearly observed that the UV-vis spectra of TPyP5-MG microgel suspensions in
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the presence of metal ions, i.e. (Na+, K+, Li+, Ag+, Ni2+, Ba2+, Co2+, Cd2+, Zn2+, Mn2+, Cu2+, Ba2+, Yb3+, Al3+, Cr3+, Gd3+, Ce3+ and La3+), are the same as that of pure TPyP5-MG microgel suspensions, as shown in Figure S4a. However, when adding Fe3+, an enhanced adsorption is observed in the wavelength range of 250 - 400 nm, which can be attributed to the adsorption of Fe3+ aqueous solution. Interestingly, with the presence of 100 µM Pb2+, the intensity of adsorption peak at 428 nm significantly decreases and a new adsorption peak at 482 nm appears (Figure S4a). The intensity ratio A482nm/A428nm of adsorptions at 482 and 428 nm is then calculated for various metal ions and shown in Figure S4b. It can be seen that for most metal ions, the A482nm/A428nm ratios are around 0.3 except of Pb2+ and Cu2+. For the case of Pb2+, the A482nm/A428nm ratio of 0.5 is significantly larger than those for other metal ions, whereas the A482nm/A428nm ratio is slightly smaller for the case of Cu2+. This significant difference of A482nm/A428nm ratio for the addition of Pb2+ and other metal ions suggests that TPyP5-MG microgel suspensions can be utilized to selectively detect Pb2+ in aqueous solutions. The response of TPyP5 DMF solution to various metal ions was further investigated. Similarly, TPyP5 DMF solution shows selective response to Pb2+. A new peak at 482 nm appears when adding Pb2+ aqueous solution into TPyP5 DMF solution at 25 °C (Figure S4c and S4d). Note that the concentration of TPyP5 in DMF solution is 6.7*10-3 mg/mL and the final concentration of metal ions is 100 µM. The response of N-MG microgel aqueous suspensions to various metal ions was also tested. It is found that that N-MGs have not response to any metal ions studied here, as shown in Figure S4e. These results indicate that the selective response of TPyP5-MG microgel suspensions to Pb2+ comes from the successful incorporation of TPyP5 into the microgel network.
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(a) 1.0
(b) 0.40 70 °C
Absorbance
0.8 0.6
0.35
0.8 0.6
A482nm
A482 nm/A428 nm
1.0
0.4
70 °C
0.4
20
30
40
50
60
70
Temperature (°C)
2+
[Pb ] 100 µM
0.25
300
(c) 1.0
400 500 600 Wavelength (nm)
700
TPyP5-MGs + Na 2+ Pb 2+ Cd + Ag 3+ Bi 3+ Cr 2+ Co 3+ La 2+ Zn 3+ Gd
0.8 0.6 0.4 0.2
25 °C TPyP5-MGs
0.15
25 °C
0.0 200
0.30
0.20
0.2
Absorbance
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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800
0
2
4
6 8 10 Time (min)
12
14
+
K 2+ Cu 2+ Mn 3+ Al 2+ Ba 3+ Ce + Li 3+ Yb 3+ Fe 2+ Ni
0.0 200
300
400 500 600 700 800 Wavelength (nm) Figure 2. (a) UV-vis spectra of TPyP5-MG microgel suspensions at various temperatures with the presence of 10 µM Pb2+. Inset shows the corresponding A482nm/A428nm ratio as a function of measuring temperature. (b) Evolution of absorption intensity at 482 nm for TPyP5-MG microgel suspensions at various temperatures with the presence of 100 µM Pb2+ as a function time. (c) UV-vis absorption spectra of TPyP5-MG microgel suspensions at 50 °C after addition of 100 µM various metal ions for 5 min. (d) The corresponding intensity ratios of A482nm/A428nm shown in (c). Inset of (d) shows the digital photographs of TPyP5-MG microgel suspensions at 50 °C without and with the presence of 100 µM Pb2+. The concentration of TPyP5-MG microgel suspensions is 0.15 mg/mL.
The effect of temperature on the Pb2+ detection performance of TPyP5-MG microgel suspensions is further investigated. Figure 2a shows the UV-vis spectra of TPyP5-MG microgel suspensions with the presence of 10 µM Pb2+ at various temperatures. Note that the concentration of TPyP5-MG microgel suspensions is 0.15 mg/mL. It can be seen that the intensity of adsorption peak at 428 nm decreases with increasing the temperature from 25 to 70 °C, whereas the intensity ACS Paragon Plus Environment
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of adsorption peak at 482 nm increases with increasing the temperature. The inset of Figure 2a shows the corresponding intensity ratio of A482nm/A428nm monotonically increases with raising temperature. The A482nm/A428nm ratio at 70 °C is about 2.5 times of that at 25 °C. Similar results are also observed with 50 µM Pb2+ (Figure S5a). Similarly, the intensity ratio of A482nm/A428nm of TPyP5 DMF solution also increases with increasing temperature with the presence of 10 µM Pb2+ (Figure S5b). These results indicate that TPyP5-MG microgel suspensions at higher temperature exhibit better detection performance for sensing Pb2+ in aqueous solution. The intensity decrease of adsorption peak at 428 nm may be attributed to the decrease of origin TPyP moieties within the TPyP5-MGs due to the chelation of TPyP moieties and Pb2+. The chelation of TPyP moieties and Pb2+ leads to the change of electronic cloud density of porphyrin chromophore and hence the appearance of new adsorption peak at 482 nm. Generally, the dissociation between TPyP moieties and Pb2+ might increase with increasing temperature, which might decrease the adsorption at 482 nm.
However, the TPyP5-MGs will shrink at high temperature so that Pb2+ ions have more
opportunities to closely contact and interact with TPyP moieties, leading to the enhanced chelation of TPyP moieties and Pb2+. The effect of enhanced chelation might overwhelm the increased dissociation at high temperature and hence result in the increase of adsorption peak at 482 nm. We further investigate the sensing kinetics of TPyP5-MGs microgel suspensions. Figure 2b shows the intensity A482nm of absorption peak at 482 nm as a function of time for TPyP5-MG microgel suspensions with the presence of 100 µM Pb2+ at various temperatures. It can be seen that for pure TPyP5-MG microgel suspensions, A482nm is unchanged, whereas A482nm increases with time and reaches the maximum value after 14 min at 25 °C. Furthermore, increasing the temperature will shorten the time required to reach the maximum value of A482nm. For example, it only needs about 5 min to reach the plateau maximum value when the temperature is above 50 °C. These results suggest that the interaction of TPyP5-MGs and Pb2+ is almost completed within 5 min for temperature above 50 °C. In other words, TPyP5-MG microgel suspensions can detect Pb2+ ACS Paragon Plus Environment
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effectively at temperature above 50 °C within 5 min. By taking the detection sensitivity, detection time and temperature together into account, the optimum condition of TPyP5-MG microgel suspensions for the detection of Pb2+ in aqueous solution can be set as 5 min and 50 °C. Figure 2c shows the UV-vis spectra TPyP5-MG microgel suspensions at 50 oC after adding 100 µM different metal ions for 5 min (i.e. Na+, K+, Li+, Ag+, Ni2+, Ba2+, Co2+, Cd2+, Zn2+, Mn2+, Cu2+, Ba2+, Yb3+, Al3+, Cr3+, Gd3+, Ce3+ and La3+, Fe3+, and Pb2+). Similar UV-vis spectra are observed as those shown in Figure S4c except of the intensities of absorption peaks at 428 nm and 482 nm. The spectrum of TPyP5-MG microgel suspensions with the presence of Pb2+ at 50 oC shows a much stronger absorption peak at 482 nm than that at 25 oC. The corresponding intensity ratio of A482nm/A428nm at 50 °C for various metal ions is shown in Figure 2d. It can be clearly seen that the A482nm/A428nm ratio of TPyP5-MG microgel suspensions with the presence of 100 µM Pb2+ at 50 °C is about 5 times of that of TPyP5-MG microgel suspensions without or with other metal ions studied here. The inset of Figure 2d shows the digital photos of TPyP5-MG microgel suspensions before and after addition of 100 µM Pb2+. Clearly, the color of microgel suspensions changes with the presence of Pb2+, which can be attributed to the appearance of absorption peak at 482 nm because of the complexation between TPyP moieties and Pb2+. These results indicate that the presence of Pb2+ (100 µM) in aqueous solution can be well selectively identified and distinguished from the rest of 19 species of other metal ions by using TPyP5-MG microgel suspensions at 50 °C.
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0.25 0.20 ∆A482nm
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0.15 [TPyP] 4.097 µM 4.544 µM 5.320 µM
0.10 0.05 0
5
10 15 2+ [Pb ]/[TPyP]
20
Figure 3. The intensity different of absorption peak at 482 nm, ∆A482nm, as a function of [Pb2+]/[TPyP] for TPyP5-MG microgel suspensions with different concentrations at 50 °C. The measurements were carried out after addition of Pb2+ for 5 min. Note that the absorption intensity at 482 nm of pure TPyP5-MG microgel suspensions was taken as the reference state and subtracted from those of TPyP5-MG microgel suspensions with the presence of various [Pb2+] to give ∆A482nm.
We further investigate the effect of concentration of TPyP5-MG microgel suspension on the interaction between TPyP moieties and Pb2+. Figure 3 shows the intensity different ∆A482nm of absorption peak at 482 nm as a function of [Pb2+]/[TPyP] for TPyP5-MG microgel suspensions with various concentrations at 50 °C, where [Pb2+]/[TPyP] represents the ratio of molar concentration for Pb2+ to TPyP moieties in the TPyP5-MG microgel suspensions. Note that the absorption intensity at 482 nm of pure TPyP5-MG microgel suspensions was taken as the reference state and subtracted from those of TPyP5-MG microgel suspensions with the presence of various [Pb2+] to give ∆A482nm. ∆A482nm first increases with increasing the concentration of Pb2+ and reaches a plateau value when [Pb2+]/[TPyP] ratio is above 10 for three concentrations of TPyP5-MG microgel suspensions studied here. At higher concentrations of [Pb2+], the interaction between TPyP moieties and Pb2+ is saturated, leading to a plateau value of ∆A482nm. Similar phenomenon was also observed for TPyP5 DMF solutions with various concentrations and the presence of various [Pb2+] at 50 °C, as shown in Figure S6.
If there is selective binding of Pb2+ to porphyrin ring, “saturation” should expect to
occur at much lower ratio. However, for TPyP5, Pb2+ might also bind to the pyridyl groups, which
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might result in the occurrence of “saturation” at high [Pb2+]/[TPyP] ratio. Nevertheless, the results of Figure 3 indicate that the concentration of TPyP5-MG microgel suspensions does not affect the interaction between TPyP moieties and Pb2+ within a certain concentration range. Ideally, one single porphyrin ring may complex with one Pb2+ ion if the efficiency of complexation is 100%, forming the so-called porphyrin lead with the presence of a lead atom in the center of the porphyrin ring. However, Figure 3 shows that it needs 10 Pb2+ ions to reach the saturation state for TPyP5-MGs at 50 °C within 5 min. The steric and hydrophobic effects as well as the immobilizing of TPyP moieties within the crosslinked networks of TPyP5-MG microgels might hinder the interaction between TPyP moieties and Pb2+, leading to the decrease of complexation efficiency and the requirement of higher concentration of Pb2+. On the other hand, the pyridine nitrogen atom of TPyP and imidazole nitrogen atom of VIM might also participate in the interaction with Pb2+, which, however do not contribute to the absorption at 482 nm. Nevertheless, the detection sensitivity of TPyP5-MG microgel suspension can reach nanomolar lever for Pb2+ in aqueous solutions (cf. Figure 4). (b) 1.6
(a) 1.0
1.4
0.8
1.2
0.6 80 µM
0.4 0.2
2+
A482nm/A428nm=0.0572*[Pb ]+0.348
1.2
2
1.0
A482nm /A428nm
A482nm/A428nm
Absorbance
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0.8 0.6 0.4
(R =0.976)
1.0 0.8 0.6 0.4 0.2 0
2
4
6
8
10 12 14 16
2+
0.0 200
0.2
0.1 µM
300
400 500 600 Wavelength (nm)
[Pb ] (µM)
700
800
0
20
40
60
80
2+
[Pb ] (µM)
Figure 4. (a) UV-vis absorption spectra of TPyP5-MG microgel suspensions with concentration of 0.15 mg/mL at 50 °C by addition of Pb2+ with various concentrations. (b) A482nm/A428nm ratio as a function of [Pb2+]. Inset of (b) shows the linear fitting of A482nm/A428nm ratio in the [Pb2+] range of 0 - 14 µM.
Figure 4a shows the UV-vis adsorption spectra of TPyP5-MG microgel suspensions with concentration of 0.15 mg/mL at 50 oC after adding Pb2+ with concentrations ranging from 0.1 to 80 ACS Paragon Plus Environment
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µM. Clearly, the intensity A428nm of absorption peak at 428 nm decreases, whereas the intensity A482nm of absorption peak at 482 nm increases with increasing the concentration of Pb2+ ([Pb2+]) at 50 °C. By plotting the ratio of A482nm/A428nm versus [Pb2+], it can be seen that A482nm/A428nm ratio first increases linearly with [Pb2+] and reaches a plateau value when [Pb2+] is larger than 16 µM, as shown in Figure 4b. A linear relationship is found to be A482nm/A428nm = 0.0572 * [Pb2+] + 0.348 with R2 value of 0.976 for the concentration range of Pb2+ from 0 to 14 µM. This linear relationship indicates that TPyP5-MG microgel suspensions can accurately detect the trace Pb2+ in aqueous solutions in the concentration range of 0 - 14 µM with an association constant of 0.0572 µM−1. The limits of detection (ܦ ) for TPyP5-MG microgel suspensions in the detection of Pb2+ can be then determined from this linear range according to the 3α IUPAC criteria:39, 49
ܦ =
ௌ್
(1)
where k is a factor with the value of 3, ܵ is the standard deviation of the blank and m is the slope of the calibration graph in the linear range. By substituting ܵ of 0.0481% obtained from five successive measurements and m of 0.0572 into the equation (1), ܦ is determined to be 25.2 nM, which is nearly 3 times lower than the safety limit of Pb2+ (i.e., ∼ 72 nM) in drinking water permitted by the EPA standard of the United States.50-51 In order to verify the reasonableness of the above-mentioned temperature selection and compare the detection of limit at different temperature,
ܦ s of TPyP5-MG microgel suspensions (0.15 mg/mL) at 30 °C and 70 °C are also determined to be about 43.4 nM and 18.7 nM for Pb2+ in aqueous solutions, respectively, as shown in Figure S7. Clearly, ܦ at 30 °C is higher than that at 50 °C. Although ܦ at 70 °C is slightly smaller than that at 50 °C, the linear relationship of A482nm/A428nm versus [Pb2+] is not good at 70 °C. Therefore, it is suitable to perform sensing measurement at 50 °C for TPyP5-MG microgel suspensions in order to sensitively detect the trace Pb2+ ion in aqueous solutions.
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(a)
(b) -6 8.0x10
4th Derivative
N=4 -5
1.0x10
4th Derivative
N=4
P2 0.0
-5
-1.0x10
350
400
450
500
550
Wavelength (nm)
80 µM
-6
4.0x10
0.0 -6
-4.0x10
N=0
-6
-8.0x10
200
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500
600
700
800
350
400
Wavelength (nm)
450 500 Wavelength (nm)
550
(c) -5 1.4x10 -5
1.2x10
-5
1.0x10
-6
8.0x10
P2
P2
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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-6
6.0x10
-6
4.0x10
1.2x10
-5
1.0x10
-5
8.0x10
-6
6.0x10
-6
4.0x10
-6
2.0x10
-6
2+
P2=7.73E-7*[Pb ]+3.23E-6 2 (R =0.958)
-2
0
2
4
6
8
10 12 14 16
2+
[Pb ] ( µ M)
-6
2.0x10
0
20
40
60
80
2+
[Pb ] (µM) Figure 5. (a) Fundamental UV-visible absorption spectra and 4th (N=4) derivative of TPyP5-MG microgel
suspensions at 50 °C with the presence of 10 µM Pb2+. Inset shows the enlargement of 4th derivative from 350 to 550 nm. The parameter, P2, is defined as the difference between the two extremums at 482 nm and 507 nm from the 4th derivative. (b) The 4th derivative of absorption spectra of TPyP5-MGs at 50 °C with the presence of various Pb2+ concentrations in the range of 350 to 550 nm. (c) The P2 values as a function of Pb2+ concentration, [Pb2+]. Note that the concentration of TPyP5-MG microgel suspensions is 0.15 mg/mL.
The detection sensitivity can be further improved by using the method of higher-order derivative spectrophotometry (HODS).52 The HODS method is mainly based on the multiple derivatives of the fundamental spectrum to obtain the corresponding curve with “fingerprinting” characteristics. Figure 5a shows the typical UV-visible absorption spectra (N = 0) and its 4th derivative (N=4) of TPyP5-MG microgel suspensions at 50 °C with the presence of 10 µM Pb2+. The difference between the two extremums at 482 and 507 nm from the 4th derivative was defined as the “fingerprinting” parameter, P2, for further analyses according to the HODS method. Figure
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5b shows the 4th derivative of absorption spectra of TPyP5-MG microgel suspensions (0.15 mg/mL) at 50 °C with the presence of various [Pb2+]. The value of P2 is then plotted versus [Pb2+] in Figure 5c. The slope value m is determined to be 7.73*10-7 by fitting to the linear concentration range from 0 to 14 µM [Pb2+]. With ܵ of 1.52*10-9, ܦ is determined by the equation (1) to be 5.9 nM, which is about 4.5 times lower than that of 25.2 nM obtained by normal method as described above and 12 times lower than the safety limit of Pb2+ (∼ 72 nM) in drinking water permitted by the EPA standard of United States.
1.2
Na +
0.9
(b)
TPyP5-MGs + Metal ions 10 µM 2+ TPyP5-MGs + Metal ions 50 µM + Pb 10 µM TPyP5-MGs 2+ TPyP5-MGs + Pb 10 µM
Li
+
+
K 3+
Al
Mn 3+
Cr
2+
Fe
Co
2+
3+
Ni
2+
Ag
Zn
2+
Cu 2+
+
Ba 2+
Cd
2+
Ce 3+
La
3+
Gd
Yb
3+
3+
Bi
Pb
sequentially adding various metal ions
1.0
Gd 3+
2+
3+
0.6 0.3
A482 nm/A428 nm
(a) 1.5 A482 nm/A428 nm
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0.8 0.6
Yb
3+
La
3+
Ce
3+ 3+
Bi
2+
Ba
2+
Zn
2+
Ni
Co
Mn 2+
2+ 3+
Cr
2+
+ 2+ Li Cu Cd Pb2+
+
K
+
3+
Na Al
+
Ag Fe3+
TPyP5-MGs
0.4 0.2
Metal ions
TPyP5-MGs
0.0
Metal ions
Figure 6. (a) A482nm/A428nm ratios of TPyP5-MG microgel suspensions (0.15 mg/mL) at 50 °C after separately adding of various metal ions for interference studies on detection performance of TPyP5-MG microgel suspensions for Pb2+ at 50 °C. The concentration of Pb2+ is 10 µM. (b) A482nm/A428nm ratios of TPyP5-MG microgel suspensions (0.15 mg/mL) at 50 °C after sequential addition of various metal ions. The addition sequence of metal ions is Yb3+, Gd3+, Ce3+, La3+, Bi3+, Ba2+, Zn2+, Ni2+, Co2+, Mn2+, Cr3+, K+, Na+, Li+, Al3+, Cu2+, Ag+, Cd2+, Fe3+ and Pb2+. Note that the concentration of metal ions is 10 µM.
Interference studies of different metal ions on detection performance of TPyP5-MG microgel suspensions for Pb2+ are also investigated. Figure 6a shows the changes of A482nm/A428nm ratios of TPyP5-MG microgel suspensions with concentration of 0.15 mg/mL at 50 oC containing 10 µM Pb2+ in the presence of various interfering ions with concentrations of 50 µM, respectively. No interference is observed for 19 species of metal ions studied here. Figure 6b shows the A482nm/A428nm
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ratios of TPyP5-MG microgel suspensions with concentration of 0.15 mg/mL at 50 oC after each addition step of various metal ions (10 µM). The sequence of adding metal ions in each step is Yb3+, Gd3+, Ce3+, La3+, Bi3+, Ba2+, Zn2+, Ni2+, Co2+, Mn2+, Cr3+, K+, Na+, Li+, Al3+, Cu2+, Ag+, Cd2+, Fe3+ and Pb2+. The co-presence of other metal ions does not affect the selectively and sensitivity of TPyP5-MG microgel suspensions toward Pb2+. The previous addition of other metal ions does not change the A482nm/A428nm ratio. However, the final addition of Pb2+ results in the significant increase of A482nm/A428nm ratio, which is consistent with the results shown in Figure 2. The TPyP5-MG microgel suspensions are then applied to detect the trace Pb2+ in real world samples. The deionized water, daily drinking water and lake water from the Yuquan campus of Zhejiang University were taken as examples. The concentrations of Pb2+ in these three samples were first determined by elemental analysis to be 0, 5.4 and 1.5 nM, respectively. It can be seen that the concentration of Pb2+ in the three samples are less than the ܦ of TPyP5-MG microgel suspensions. Given concentrations of Pb2+, i.e. 1, 5, and 10 µM, were then spiked into the three samples, respectively. The concentrations of Pb2+ in the spiked samples were then determined by elemental analysis and UV-visible absorption using TPyP5-MG microgel suspensions (0.15 mg/mL), respectively. The results of Table 1 show that the concentrations of Pb2+ in the three samples determined by these two different methods are consistent with each other and also agree with the amounts of Pb2+ arbitrarily spiked in these samples. These results suggest that the TPyP5-MG microgel suspensions can be potentially applied for sensing trace Pb2+ in real samples.
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Table 1. The concentrations of Pb2+ in deionized water, daily drinking water and lake water from the Yuquan campus of Zhejiang University as determined by elemental analysis and UV-vis absorption using TPyP5-MG microgel suspensions (0.15 mg/mL), respectively.
Sample Deionized water
Drinking Water
Lake Water
Elemental
[Pb2+]
UV-vis
UV-vis
Analysis
Spiked
(Normal)
(HODS)
0 nM
1 µM
996 ± 15 nM
990± 18 nM
994 ± 15 nM
5 µM
4992 ± 10 nM
4997 ± 24 nM
5006 ± 21 nM
10 µM
10001 ± 12 nM
9996 ± 22 nM
10008 ± 18 nM
1 µM
1029 ± 36 nM
1018 ± 26 nM
1012 ± 31 nM
5 µM
5017 ± 22 nM
5019 ± 17 nM
5020 ± 25 nM
10 µM
9997 ± 12 nM
10017 ± 20 nM
10006 ± 16 nM
1 µM
990 ± 26 nM
1010 ± 22 nM
1003 ± 18 nM
5 µM
5023 ± 28 nM
4993 ± 23 nM
4993 ± 30 nM
10 µM
9989 ± 25 nM
9989 ± 29 nM
10007 ± 22 nM
5.40 nM
1.5 nM
Elemental Analysis
CONCLUSIONS. Tetra(4-pyridyl)porphyrin functionalized thermosensitive ionic microgels (TPyP5-MGs) were successfully obtained, which have hydrodynamic radius of about 189 nm at 25 oC with uniform size distribution and exhibit thermosensitive character in aqueous suspensions. The TPyP5-MG microgel suspensions can optically response to trace Pb2+ ions in aqueous solution with high sensitivity and selectivity over the interference of other 19 species of metal ions, namely Yb3+, Gd3+, Ce3+, La3+, Bi3+, Ba2+, Zn2+, Ni2+, Co2+, Mn2+, Cr3+, K+, Na+, Li+, Al3+, Cu2+, Ag+, Cd2+, and Fe3+. The limit of detection for TPyP5-MG microgel suspensions with concentration of 0.15 mg/mL in detecting Pb2+ at 50 oC is about 25.2 nM, which can be improved to be 5.9 nM by HODS. Such TPyP5-MG microgel suspensions might have potential application for the detection of trace Pb2+ in real world samples.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XX.XXXX/acsami.XXXXXXX. ACS Paragon Plus Environment
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Possible structures of TPyP5 precursor, 1H-NMR and MS spectra of TPyP5 precursor, standard calibration curve of TPyP in DMF, additional UV-vis absorption spectra of TPyP5 precursor and TPyP5-MG microgel aqueous suspensions with the presence of various metal ions at different temperature.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected].
ORCID Binyang Du: 0000-0002-5693-0325 Xianjing Zhou: 0000-0001-7703-7555
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (Nos. 21674097 and 21322406), the second level of 2016 Zhejiang Province 151 Talent Project, and Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry (201601), Changchun Institute of Applied Chemistry, Chinese Academy of Sciences for financial support.
REFERENCES (1) Li, Y.-H.; Di, Z.; Ding, J.; Wu, D.; Luan, Z.; Zhu, Y. Adsorption Thermodynamic, Kinetic and Desorption Studies of Pb2+ on Carbon Nanotubes. Water Res. 2005, 39 (4), 605-609. (2) Biasino, J.; Domínguez, J. R.; Alvarado, J. Hydrogen Peroxide in Basic Media for Whole Blood Sample Dissolution for Determination of Its Lead Content by Electrothermal Atomization Atomic Absorption Spectrometry. Talanta 2007, 73 (5), 962-964. (3) Liang, P.; Sang, H. Determination of Trace Lead in Biological and Water Samples with
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