Letter pubs.acs.org/ac
Real-Time, Selective Detection of Copper(II) Using Ionophore-Grafted Carbon-Fiber Microelectrodes Yuanyuan Yang,† Ahmad A. Ibrahim,† Parastoo Hashemi,*,‡ and Jennifer L. Stockdill*,† †
Department of Chemistry, Wayne State University, Detroit, Michigan 48202, United States Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States
‡
S Supporting Information *
ABSTRACT: Rapid, selective detection of metals in complex samples remains an elusive goal that could provide critical analytical information for biological and environmental sciences and industrial waste management. Fast-scan cyclic voltammetry (FSCV) using carbon-fiber microelectrodes (CFMs) is an emerging technique for metal analysis with broad potential applicability because of its rapid response to changes in analyte concentration and minimal disturbance to the analysis medium. In this communication, we report the first effective application of covalently modified CFMs to achieve highly selective, subsecond Cu(II) measurements using FSCV. A two-part strategy is employed for maximum selectivity: Cu(II) binding is augmented by a covalently grafted ionophore, while binding of other metals is prevented by chemical blocking of nonselective surface adsorption sites. The resulting electrodes selectively detect Cu(II) in a complex medium comprising several interfering metals. Overall, this strategy represents a transformative innovation in real-time electrochemical detection of metal analytes.
M
by chemically capping surface oxygen functionalities and (2) promote selective adsorption of the target analyte by appending the CFM with an ionophore selective for that metal. A bulky silyl group was selected as the capping group for the surface hydroxyl groups.25 To attach the ionophore to the CFM, we planned an azide−alkyne cycloaddition of an azido-substituted ionophore to aryl alkynes on the CFM surface.26 We chose to investigate this strategy in the context of Cu(II) analysis because of the broad interest in rapid copper detection across a range of disciplines.27−30 We identified Cu(II) ionophore 1, a commercially available ionophore, as an excellent candidate structure for rendering Cu(II) selectivity to CFMs (Scheme 1).31,32 However, to accomplish covalent modification, we required a chemically modified version of ionophore 1 possessing an azide, which could react with alkyne scaffolding groups. Thus, azido-ionophore 7 was synthesized from commercially available 3-nitrophthalic acid (2). Selective borane reduction of the carboxylic acids was accomplished in quantitative yield, generating diol 3. Hydrogenolysis of the nitro group yielded aniline derivative 4, which was converted to the corresponding azide (5) in the presence of trimethylsilyl azide and t-butyl nitrite. Bromination of the benzylic alcohols afforded dibromide 6, which was treated with a prestirred
ethods for the rapid, selective, and sensitive detection of metals (for a general review, see refs 1−6) are highly desirable for a number of applications including analysis in medical,7,8 biological,9−11 environmental,12−15 and industrial16−18 systems. Electrochemical methods offer a complementary approach to imaging techniques for metal detection. Ionselective electrodes have shown promise for selective electrochemical metal analysis; however, they suffer from low stability and their response time does not afford information on a time scale rapid enough to investigate systems where metal levels fluctuate dynamically.19−21 An attractive new alternative is fastscan cyclic voltammetry (FSCV) at carbon-fiber microelectrodes (CFMs).22−24 FSCV takes advantage of metal adsorption onto the striated, highly oxygenated CFM surface to enable subsecond metal detection. The dimensions of the CFM (typically ∼7 μm × 150 μm) are minimally disturbing to the analysis medium. However, to date, application of this technique to complex samples has been hindered by limited selectivity. Namely, the oxygen moieties on the CFM surface, which are responsible for analyte adsorption,24 do not discriminate well between different metal cations. Some selectivity is inherent to voltammetry because redox peaks occur at discrete potentials. However, in a complex mixture, the signal of interest is often masked by overlapping or large signals due to nonselective adsorption to the CFM surface. To overcome this limitation, we sought to render selective the adsorption of metals onto CFMs. We envisioned a two-part CFM modification strategy: (1) prevent nonselective adsorption © XXXX American Chemical Society
Received: March 1, 2016 Accepted: April 15, 2016
A
DOI: 10.1021/acs.analchem.6b00825 Anal. Chem. XXXX, XXX, XXX−XXX
Letter
Analytical Chemistry
FSCV analysis for analytes that adsorb to the CFM surface. CFM 2 (Column II) shows no significant electrochemical signal, indicating successful capping of the oxygen adsorption sites on the electrode. Initial tests of CFM 3 showed minimal signal for Cu(II) (data not shown). We reasoned that because of the spacing between the aryl alkyne scaffolding groups, there should be a significant decrease in adsorption sites for Cu(II) compared to an unmodified electrode bearing many oxygen functionalities. To increase the number of adsorption sites available to Cu(II), we increased the length of the electrode from 150 to 300 μM.34 The functionalization process in Scheme 2 was repeated. After this adjustment, CFM 3 showed a very clear reduction peak at −0.9 V upon exposure to 1.0 μM Cu(II) (Figure 1, Column III). This shift to a more negative potential is consistent with the more thermodynamically favorable interaction expected for the soft Lewis acid Cu(II) with the soft Lewis basic sulfur moieties in the ionophore. We then turned our attention to selective detection of Cu(II) as the minor component of a complex sample (1.0 μM Cu(NO3)2 and 10 μM each of Zn(NO3)2, Cd(NO3)2, Ni(NO3)2, Co(NO3)2, Ca(NO3)2, Mg(NO3)2, Pb(NO3)2, and Mn(NO3)2).35 As shown in Figure 2, unfunctionalized CFM 1 and O-blocked, ionophore-appended CFM 3 were each exposed to the mixed metal solution via flow injection analysis. It is impossible to distinguish any recognizable Faradaic features in the CV produced by CFM 1 in the mixed metal sample. In contrast, CFM 3 showed characteristic Faradaic behavior with a strong reduction peak at −0.9 V.36 To our delight, there was no apparent signal arising from any of the other metals in the solution. Thus, our ionophore-grafted, O-blocked electrodes are able to detect Cu(II) as the minor component of a highly interfering mixture of metal ions. Notably, this data represents the first time that FSCV of a complex sample has resulted in a selective response to one metal analyte. Furthermore, this is the first time that Cu(II) has been selectively detected with subsecond temporal resolution at a microsensor. The temporal resolution of FSCV is 100 ms, and this holds for our Cu(II) measurement. The response profile, shown in the bottom panel of Figure 3 (current vs time at peak cathodic position) is not an ideal step response, as is typically the case for FSCV. These deviations from the ideal response, including the tailing or streaking effect, can be modeled via deconvolution algorithms37 and kinetic calibrations,38 and this is the focus of our ongoing work. Table 1 below compares the response time and limit of detection (LOD) for our method as compared with other common Cu(II)-selective electroanalytical methods.39−44 Our work represents both the fastest response time and the lowest LOD. Thus, it is clear that the rapid response does not come at the expense of sensitivity. The stability of this modification is a critical parameter to address. Thus, we exposed 4 ionophore-grafted, O-blocked electrodes (CFM 3) to 50 successive injections of Cu(II). Figure 4A shows the averaged, normalized current for these 4 electrodes with each injection. The electrodes show excellent stability over this short-term experiment. In Figure 4B, electrode shelf life was assessed. Clean, dry electrodes were stored for 16 weeks. Measurements were taken weekly for the first 4 weeks and monthly after that. The averaged, normalized current at these specific time points was consistent, indicating that the ionophore does not undergo any decomposition upon storage over this time period.
Scheme 1. Synthesis of Azido-Ionophore (Compound 7)
solution of CS2, diisobutylamine, and K2CO3 in MeOH to provide the desired azide-appended ionophore 7.33 As shown in Scheme 2, scaffolded CFM 1 was exposed to a solution of t-butyldimethylsilyl chloride in the presence of N,Ndimethylaminopyridine and imidazole to convert any hydroxyl groups present to the corresponding silyl ethers, which are known to be poor chelators of metal ions. This process was conducted while applying a negative potential to the electrode with the aim of reducing any carbonyl groups that might be present to the corresponding alcohols in situ. The resulting alcohols were also capped with silyl groups, affording Oblocked CFM 2. This step was executed at 0, −1, and −1.9 V. The lowest current (i.e., most efficient capping) was observed at −1.9 V.33 Azide−alkyne cycloaddition afforded ionophoregrafted electrode CFM 3. Scheme 2. Inhibition of Excess Oxygenated Groups of CFM
To validate the efficacy of each of the 2 parts of our strategy for selectivity, each electrode was first characterized by its response to a 1.0 μM Cu(NO3)2 solution (Figure 1). Background subtracted cyclic voltammograms (CVs) taken every 100 ms are displayed as potential on the y-axis, time on the x-axis, and current in false color according to the scale indicated. A representative cyclic voltammogram (CV) is shown below each color plot. These were taken at the time indicated by the white dotted lines in the color plots. As we anticipated, CFM 1 shows a characteristic reduction peak for Cu(II) at −0.7 V (Column I; see white arrow in color plot and blue circle on CV). The small peaks at the switching potentials occur due to capacitive changes on the electrode surface related to Cu(II) adsorption. These peaks commonly occur during B
DOI: 10.1021/acs.analchem.6b00825 Anal. Chem. XXXX, XXX, XXX−XXX
Letter
Analytical Chemistry
Figure 1. In 1.0 μM Cu(II) solution, (I) scaffolded, O-blocked electrodes show no signal and (II) Cu(II)-ionophore-functionalized, O-blocked electrodes show expected color plot and CV for Cu(II). Waveform: −1.2 V/+0.8 V. Exposed carbon fiber length: (I) 150 μm, (II) 150 μm, and (III) 300 μm.34
Figure 2. In mixed metal solution, (I) unfunctionalized electrodes show non-Faradaic behavior, and (II) Cu(II)-ionophore-functionalized, O-blocked electrodes show clean Cu(II) redox signal. Waveform: −1.2 V/+0.8 V. Exposed carbon fiber length: (I) 150 μm and (II) 300 μm.34
In summary, we have developed the first effective strategy for covalent modification of CFMs with ionophores, enabling realtime selective detection of Cu(II). This work is highlighted by the design and efficient synthesis of the azido-ionophore 7 and by a two-part strategy for selectivity that includes capping
surface hydroxyl groups to prevent nonselective adsorption of other metals while selectively coordinating Cu(II). The resulting electrodes are highly selective for Cu(II) in a solution of 8 other divalent metal ions at 10-fold higher concentrations per metal. This sensor represents a transformative advancement C
DOI: 10.1021/acs.analchem.6b00825 Anal. Chem. XXXX, XXX, XXX−XXX
Analytical Chemistry
■
Letter
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.6b00825. Synthetic, electrochemical, and spectroscopic methods. (PDF)
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
Figure 3. Top: Cu(II) ionophore-functionalized electrode response as color plot in mixed metal solution. Bottom: Current vs time profile taken at peak cathodic potential from the color plot as denoted by the horizontal white dashed line.
ACKNOWLEDGMENTS The authors would like to thank Wayne State University and the University of South Carolina for generous financial support (startup funds to J.L.S. and P.H., Rumble Fellowship to Y.Y.) and are thankful for the Eli Lilly Young Investigator Award to P.H. We gratefully acknowledge the staff of the WSU Lumigen Instrument Center for spectroscopic support. We also would like to thank Srimal Samarnayake for generating the TOC artwork.
Table 1. Comparison of LOD and Response Time of Our Method with Other Commonly Used Methods for Cu(II)Selective Electroanalysis sensor specifics Orion Cupric Electrode, Thermo Scientific No. 9629BNWP Cu Ion-Selective Electrode, Metrohm No. 6.0502.140 ref 39 ref 40 ref 41 ref 42 ref 43 ref 44 this work
methodology
limit of detection, M
potentiometry
1 × 10−8
potentiometry potentiometry potentiometry voltammetry voltammetry potentiometry potentiometry FSCV
response time, s
1 × 10−8
10
× 10−9 × 10−6 × 10−9 × 10−6 × 10−8 × 10−6 × 10−9
9 20 300 20 14 5−10 0.1
6.3 2.0 5.3 1 1 3.2 5
■
15
REFERENCES
(1) Carter, K. P.; Young, A. M.; Palmer, A. E. Chem. Rev. 2014, 114, 4564−4601. (2) Que, E. L.; Domaille, D. W.; Chang, C. J. Chem. Rev. 2008, 108, 1517−1549. (3) March, G.; Nguyen, T. D.; Piro, B. Biosensors 2015, 5, 241−275. (4) Aragay, G.; Pons, J.; Merkoçi, A. Chem. Rev. 2011, 111, 3433− 3458. (5) Li, M.; Gou, H.; Al-Ogaidi, I.; Wu, N. ACS Sustainable Chem. Eng. 2013, 1, 713−723. (6) Buffle, J.; Tercier-Waeber, M.-L. TrAC, Trends Anal. Chem. 2005, 24, 172−191. (7) Hill, D. S.; Cabrera, R.; Schultz, D. W.; Zhu, H.; Lu, W.; Finnell, R. H.; Wlodarczyk, B. J. Behav. Neurobiol. 2015, 2015, 426263. (8) Rahbar, M. H.; Samms-Vaughan, M.; Dickerson, A. S.; Loveland, K. A.; Ardjomand-Hessabi, M.; Bressler, J.; Shakespeare-Pellington, S.; Grove, M. L.; Pearson, D. A.; Boerwinkle, E. Int. J. Environ. Res. Public Health 2015, 12, 83−105. (9) Xiang, Y.; Lu, Y. Inorg. Chem. 2014, 53, 1925−1942. (10) Bagchi, P.; Morgan, M. T.; Bacsa, J.; Fahrni, C. J. J. Am. Chem. Soc. 2013, 135, 18549−18559. (11) Kim, B.; Jung, I. H.; Kang, M.; Shim, H.-K.; Woo, H. Y. J. Am. Chem. Soc. 2012, 134, 3133−3138. (12) Zhou, X.; Nie, J.; Du, B. ACS Appl. Mater. Interfaces 2015, 7, 21966−21974. (13) Wei, Q.; Nagi, R.; Sadeghi, K.; Feng, S.; Yan, E.; Ki, S. J.; Caire, R.; Tseng, D.; Ozcan, A. ACS Nano 2014, 8, 1121−1129. (14) Zhang, L.; Chang, H.; Hirata, A.; Wu, H.; Xue, Q.-K.; Chen, M. ACS Nano 2013, 7, 4595−4600. (15) López Marzo, A. M.; Pons, J.; Blake, D. A.; Merkoçi, A. Anal. Chem. 2013, 85, 3532−3538. (16) Gumpu, M. B.; Sethuraman, S.; Krishnan, U. M.; Rayappan, J. B. B. Sens. Actuators, B 2015, 213, 515−533. (17) Burkhead, J. L.; Gogolin Reynolds, K. A.; Abdel-Ghany, S. E.; Cohu, C. M.; Pilon, M. New Phytol. 2009, 182, 799−816. (18) Valipour, A.; Hamnabard, N.; Woo, K.-S.; Ahn, Y. H. J. Environ. Manage. 2014, 145, 1−8. (19) Liang, R.; Yin, T.; Qin, W. Anal. Chim. Acta 2015, 853, 291− 296.
Figure 4. Stability tests of ionophore-grafted electrodes with blocked surface oxygen groups. (A) Response of CFM 3 to 50 successive injections of 1.0 μM Cu(II). (B) Response of CFM 3 to injections of 1.0 μM Cu(II) over 16 weeks.
for the field of electroanalysis in providing unparalleled selectivity and temporal resolution. Efforts to employ this approach in the selective detection of other metal species are ongoing. D
DOI: 10.1021/acs.analchem.6b00825 Anal. Chem. XXXX, XXX, XXX−XXX
Letter
Analytical Chemistry (20) Jasiński, A.; Guziński, M.; Lisak, G.; Bobacka, J.; Bocheńska, M. Sens. Actuators, B 2015, 218, 25−30. (21) Zuliani, C.; Diamond, D. Electrochim. Acta 2012, 84, 29−34. (22) Pathirathna, P.; Yang, Y. Y.; Forzley, K.; McElmurry, S. P.; Hashemi, P. Anal. Chem. 2012, 84, 6298−6302. (23) Yang, Y. Y.; Pathirathna, P.; Siriwardhane, T.; McElmurry, S. P.; Hashemi, P. Anal. Chem. 2013, 85, 7535−7541. (24) Pathirathna, P.; Samaranayake, S.; Atcherley, C. W.; Parent, K. L.; Heien, M. L.; McElmurry, S. P.; Hashemi, P. Analyst 2014, 139, 4673−4680. (25) Heien, M. L. A. V.; Phillips, P. E. M.; Stuber, G. D.; Seipel, A. T.; Wightman, R. M. Analyst 2003, 128, 1413−1419. (26) Yang, Y.; Ibrahim, A. A.; Stockdill, J. L.; Hashemi, P. Anal. Methods 2015, 7, 7352−7357. (27) (a) Li, H.; Zhang, P.; Smaga, L. P.; Hoffman, R. A.; Chan, J. J. Am. Chem. Soc. 2015, 137, 15628−15631. (28) Choi, Y.-A.; Keem, J. O.; Kim, C. Y.; Yoon, H. R.; Heo, W. D.; Chung, B. H.; Jung, Y. Chem. Sci. 2015, 6, 1301−1307. (29) Carter, K. P.; Young, A. M.; Palmer, A. E. Chem. Rev. 2014, 114, 4564−4601. (30) Cotruvo, J. A., Jr.; Aron, A. T.; Ramos-Torres, K. M.; Chang, C. J. Chem. Soc. Rev. 2015, 44, 4400−4414. (31) CAS# [125769-67-7]. (32) Kamata, S.; Murata, H.; Kubo, Y.; Bhale, A. Analyst 1989, 114, 1029−1031. (33) See the Supporting Information for details. (34) When a 300 μm bare electrode was used, the signal overloaded the instrument, so the 150 μm electrode data is provided instead. (35) In this mixture, there is a 1:80 ratio of Cu(II) to all other divalent metal ions. (36) The streaking behavior in the color plot for CFM 3 indicates a prolonged clearance time for Cu(II) relative to the scaffolded electrode (CFM 1). This effect is typical in FSCV and can be accounted for with development of deconvolution and kinetic models. (37) Venton, B. J.; Troyer, K. P.; Wightman, R. M. Anal. Chem. 2002, 74, 539−546. (38) Atcherley, C. W.; Laude, N. D.; Monroe, E. B.; Wood, K. M.; Hashemi, P.; Heien, M. L. ACS Chem. Neurosci. 2015, 6, 1509−1516. (39) Singh, A. K.; Sahani, M. K.; Bandi, K. R.; Jain, A. K. Mater. Sci. Eng., C 2014, 41, 206−216. (40) Ansari, R.; Delavar, A. F.; Aliakbar, A.; Mohammad-Khah, A. J. Solid State Electrochem. 2012, 16, 869−875. (41) Ashkenani, H.; Taher, M. A. J. Electroanal. Chem. 2012, 683, 80−87. (42) Lee, S. H.; Sumranjit, J.; Tongkate, P.; Chung, B. H.; Lee, H. J. Electrochim. Acta 2014, 123, 198−204. (43) Baig, U.; Khan, A. A. J. Ind. Eng. Chem. 2015, 29, 392−399. (44) Wardak, C.; Lenik, J. Sens. Actuators, B 2013, 189, 52−59.
E
DOI: 10.1021/acs.analchem.6b00825 Anal. Chem. XXXX, XXX, XXX−XXX
