Electrochemical detection of dopamine by a calixarene-cellulose acetate mixed Langmuir-Blodgett monolayer
Graphical abstract
Introduction
Electrochemistry as an analytical approach exhibits high sensitivity; however, it usually lacks selectivity. The latter is often increased by modifying the electrode surface with mostly organic molecules that can bind selectively the desired analyte. Calixarene, a macrocycle oligomer of p-substituted phenol, is widely used for sensing, due to its unique barrel type structure, which provides selective interactions with various guest molecules [1]. Specifically, calixarene films have shown high selectivity towards dopamine (DA) by decreasing the accessibility of common electroactive interferences to the electrode surface. The specific interaction between calixarene and DA was utilized for the first time for assembling an amperometric sensor by Wang et al. [2]. Since then, the derivatives of calixarene [[3], [4], [5]] were also reported for the selective detection of DA. The composite of calixarene and graphene based materials showed a synergetic effect and improved sensitivity [6,7]. The effect of functionalizing calixarene with different groups, primarily by alkanethiols, and ring size, on its association with DA was described [8,9]. An LOD of 4.9 pM of DA with a linear range of 10−11-10−6 M using calix [4]arene mercaptoalkyl derivatives adsorbed on a gold electrode, was reported by Kurzątkowska et al. [8]. However, the functionalization of calixarene is not trivial and the synthesis of either thiolated calixarene or the incorporation of it as part of a polymer requires significant efforts.
The Langmuir–Blodgett (LB) technique is an appealing approach for assembling calixarene because it can, in principle, induce ordering and increase the density of calixarene units per area by both compression and formation of multilayers. Yet, such layers need to maintain direct electron transfer between the target molecule associated with calixarene and the electrode. We are aware of only two studies [10,11], which discussed the interaction between DA and calixarene or resorcinarene LB films. Weis and coworkers [10] reported that increasing the concentration of DA in the subphase affected the organization of calix [4]resorcinarene Langmuir layer, and the binding between the two species was highly related to the resorcinarene orientation. Wang et al. [11] proposed a LB film of resorcinarene modified pretreated glassy carbon electrode (GCE) as an electrochemical sensor for simultaneous determination of DA and uric acid (UA). The synergetic effects of C-undecylcalix [4]resorcinarene LB film on a pretreated GCE resulted in high sensitivity with detection limits of 20 and 80 nM for DA and UA, respectively.
Calix [6]arenes were reported [12] to form stable and well-ordered Langmuir monolayers. However, in fact, compared with the theoretical molecular area of calix [6]arenes, the observed experimental area per molecule was higher [13], indicating possible molecular reorientation at the air-water interface. The reorientation hypothesis has been further analyzed by molecular modeling [[14], [15], [16]], revealing that the calix [6]arenes tend to be tilted and aggregate at the interface along with the formation of dimers [15]. This would not only decrease the available sensing unit per area, but also reduce the accessibility of the analyte, e.g., DA, within the calix [6]arene layer. Therefore, stabilization of the calix [6]arene by incorporating with another species within the Langmuir film is envisioned to improve the sensing performance.
Cellulose acetate, CA, is widely utilized in Langmuir and LB films [17,18] due to its low solubility in water and high stability [19]. Moreover, the strong and steady interaction between CA and calix [20,21] provides the basis for CA to act as a perfect stabilization matrix to enhance the monomer arrangement within the LB monolayer.
Herein, we developed a mixed cellulose acetate and calix [6]arene LB film, CA-calix, for the determination of DA. The effects of the number of LB layers deposited on an Au electrode, the ratio between the calixarene and CA on the electroanalytical performance of the electrode were investigated and optimized. The mixed CA-calix LB layer exhibits better affinity towards DA as compared with the pure CA or calix films. Two linear ranges were obtained, with LODs of 2.54 nM within 0.1–100 nM and 0.163 μM within 0.1–7.5 μM.
Section snippets
Chemicals and solutions
4-tert-butylcalix [6]arene (calix, C66H84O6, 95%) and cellulose acetate (CA, average Mñ30,000 with acetyl content of 40%) were obtained from Aldrich and used as received. Solutions of calix and CA were prepared in chloroform (CHCl3, HPLC analyzed, J.T. Baker) with a concentration of 0.5 mg mL−1, respectively. Phosphate buffered saline (PBS, 0.01 M, pH 7.4, at 25 °C) was prepared by dissolving phosphate buffered saline powder (MDL number MFCD00131855) in 1 L deionized water. HCl (J. T. Baker), H2
Results and discussion
The sensitive and reproducible detection of DA is of great importance for the early diagnosis of the central nervous system (CNS) dysfunctions [22], such as Parkinson's and Alzheimer's diseases [23] and attention deficit hyperactivity disorder [24]. Among all methods for the DA determination, electroanalytical techniques are highly valued for their sensitivity, simplicity and practicability [25,26]. Nevertheless, the close oxidation potentials make it difficult to separate the signal of DA,
Conclusions
A novel approach to increase the electrochemical sensing performance using Langmuir-Blodgett films is proposed. The approach is based on co-spreading a mixture of the main component constituting the Langmuir film with the selectivity element. Specifically in this work, the Langmuir film is made of cellulose acetate, while the selectivity factor is 4-tert-butylcalix [6]arene. The latter is known for its selectivity towards a variety of small functionalized aromatic rings, such as dopamine. The
Acknowledgement
This work was supported by the China-Israel Cooperative Scientific Research (Israeli contract 3-13557) and (Chinese contract 2016YFE0129900). The Harvey M. Krueger Family Centre for Nanoscience and Nanotechnology of the Hebrew University is acknowledged.
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