Plasma polyacrylic acid and hollow TiO₂ spheres modified with rhodamine B for sensitive electrochemical sensing Cu(II)

Abstract

We report a novel nanocomposite of hollow TiO₂ microspheres and plasma polyacrylic acid (TiO₂@PPAA) modified with rhodamine B (TiO₂@PPAA-RhB). This nanocomposite was further used as an electrochemical sensor for the ultra-sensitive and selective detection of Cu²⁺ in water. A gold electrode was modified with hollow TiO₂ nanospheres, followed by the closely bonded layer of PPAA via plasma polymerization. Subsequently, the carboxyl groups of TiO₂@PPAA nanocomposites were activated to ester groups by N-hydroxysuccinimide/1-ethyl-3-(3-dimethylaminoprolyl) carbodiimide hydrochloride. The resultant activated ester groups reacted with the amino groups on rhodamine to form the amide groups, leading to the modification of TiO₂@PPAA nanocomposites with rhodamine. Considering the synergistic effect of strong electrostatic interaction between the carboxyl groups of PPAA and Cu²⁺, the direct adsorption of Cu²⁺ on the TiO₂ surface, and the coordination chemistry formed between Cu²⁺ and rhodamine, the developed electrochemical biosensor exhibited high sensitivity of Cu²⁺ detection, with a detection limit of 0.404 pM within the range of 0.001–10 nM of Cu²⁺. Therefore, the fabricated electrochemical sensor based on TiO₂@PPAA-RhB can be obtained via plasma polymerization and represents a highly promising tool for use in environmental monitoring.

---

1 Introduction

Metal ion recognition and sensing have received considerable attention because of their important roles in biological, industrial, and environmental processes. Among the essential heavy metal ions in the human body, Cu²⁺ is the third most abundant after Fe³⁺ and Zn²⁺; Cu²⁺ plays important roles in several biological processes.¹ The toxicity of copper ions for humans is rather low compared with other heavy metals, but certain microorganisms are affected by even submicromolar Cu²⁺ concentrations.² Consequently, effective Cu²⁺ detection in water or physiological samples is of toxicological and environmental importance.³ To date, several elaborate approaches, including fluorescence chemosensors,⁴ atomic absorption spectrometry,⁵ inductively coupled plasma atomic emission spectrometry,⁶ and electrochemical strategies,⁷ have been established for the determination of Cu²⁺. Electrochemical methods have recently attracted more attention because of their low cost, simple instrumentation, and easy to real-time and in situ detection.⁸

Polyacrylic acid (PAA) is composed of numerous carboxyl groups on the line type macromolecular chains, thereby giving the rapid contact between heavy metal ions and carboxyl groups, and more binding sites at the interface.⁹ Recently, a number of researchers have investigated the adsorption of heavy metal ions onto linear or cross-linked PAA via the electrostatic interaction and specific complex formation. For example, PAA was used to modify the inorganic or polymeric adsorbent,¹⁰ such as magnetic mesoporous carbon,¹¹ poly(glycidyl methoarylate),¹² and polyhyaluronic acid¹³ with carboxyl-containing species, hence providing these materials with high adsorption ability of Cd²⁺, Cu²⁺, and Ca²⁺. The performance of heavy metal ion removal is improved by modifying graphene oxide and Fe₃O₄ nanocomposites with PAA and are used as new adsorbents, thus showing excellent recyclable removal of Pb²⁺, Cu²⁺, and Cd²⁺ ions from aqueous solution.¹⁴ PAA can also be prepared via plasma polymerization, in which plasma deposition affords strongly adherent pin-hole free film and functionalizes stent surfaces with high density of grafting sites that facilitate covalent attachment of biopharmaceuticals.¹⁵ Plasma polyacrylic acid (PPAA) has been studied to obtain coatings with a high density of carboxylic acid groups. Such carboxyl-group surfaces can have important industrial applications, such as improving adhesion and aging resistance of bonded components in the automotive industry and activating the polyolefin surface, thus replacing conventional treatments in the packing sector.¹⁶ However, PAA-related films are seldom used as the sensitive layer of electrochemical biosensors for detecting heavy ions, mainly because of the non-specific adsorption between the heavy metal ions and PAA and their poor electrochemical performance.¹⁷

Additionally, rhodamine B (RhB) derivatives are nonfluorescent and colorless, whereas ring opening of the corresponding spirolactam gives rise to strong fluorescence emission and a pink color.¹⁸ A spirolactam (nonfluorescent) to ring-opened amide (fluorescent) process was utilized for metal ion detection. Most of these sensors of rhodamine-based derivatives bearing a pyrene group as a chemosensor for Cu²⁺ were reported, in which the rhodamine ring-opening process was introduced to provide a colorimetric change and “turn-on” fluorescence signal toward Cu²⁺.¹⁹ Pyrene fluorescence is quenched upon Cu²⁺ addition; thus, the signal is not observed. Introducing another fluorophore to provide a strong fluorescence first upon binding with some other ions and show typical rhodamine fluorescence later by Cu²⁺ replacement could ensure ratiometric output signals.²⁰ However, as an organic pollutant in the environment, the residue of rhodamine dyes threatens human health. Treating wastewater containing rhodamine produced through the normal determination method of heavy metal ions is difficult.²¹ Therefore, developing a much more convenient sensor based on rhodamine using a feasible method, which can be easily used in the natural environment and recycled after being used, is highly desirable.

TiO₂ microsphere is a useful and functional material in the fields of dye-sensitized solar cells, photocatalysts, and environmental treatments because of its semiconductive properties, physical and chemical stabilities, and cost-effective fabrication.²² Metals, particularly heavy metals, are considered to be health risks to both humans and the ecosystem because of their toxicity. Hu et al. investigated the desorption of Pb(II), Cu(II), and Zn(II) from TiO₂ nanoparticles.²³ Similarly, Gao et al.²⁴ conducted an experiment about the adsorption mechanism and behavior of Pb(II) and Cd(II) ions on different nanosized TiO₂ crystals. More importantly, TiO₂ leads to a high host–guest interface with improved light adsorption and photoactivity. Therefore, TiO₂ is highly potential for adsorption of heavy metal materials. Especially, hollow TiO₂ spheres (h-TiO₂) exhibit highly photocatalytic and adsorption activity due to the high surface area and porous nanostructure.²⁵

Combining the high Cu²⁺ adsorption of PPAA, selective interaction between Cu²⁺ and RhB, and high electrochemical performance of h-TiO₂ spheres, the present work aims to investigate a novel nanocomposite of h-TiO₂ microspheres and plasma polyacrylic acid (h-TiO₂@PPAA) modified with rhodamine B (TiO₂@PPAA-RhB). The resultant nanocomposite was further used as the electrochemical sensor for the ultra-sensitive detection of Cu²⁺ (Scheme 1). Compared with conventional methods, the present strategy have two advantages: (i) relatively good electrochemical properties because of the presence of h-TiO₂ microspheres and (ii) high selectivity of Cu²⁺ detection caused by the high density of rhodamine in the 3D network of TiO₂@PPAA-RhB.

Scheme 1 Schematic diagram of the developed electrochemical sensor for detecting Cu²⁺ ions based on h-TiO₂@PPAA-RhB, including (I) preparation of h-TiO₂@PPAA, (II) activation of h-TiO₂@PPAA by EDC/NHS, (III) modification of h-TiO₂@PPAA with rhodamine, and (IV) detection of Cu²⁺ ions using the developed sensor based on h-TiO₂@PPAA-RhB.

2 Experimental section

2.1 Chemicals and materials

Acrylic acid, N-hydroxysuccinimide (NHS), 1-ethyl-3-(3-dimethylaminoprolyl) carbodiimide hydrochloride (EDC), n-butyl titanate, RhB, and anhydrous glucose were obtained from Aladdin (Shanghai, China). K₄[Fe(CN)₆]·H₂O and K₃[Fe(CN)₆] were purchased from Alfa Aesar. All other reagents used were of analytical grade and employed as received.

2.2 Solutions

Phosphate buffer solution (PBS,0.1 M, pH 7.4) was prepared by mixing the stock solution of 0.1 M NaH₂PO₄ and 0.1 M K₂HPO₄ with the proportion of v(Na₂HPO₄) : v(KH₂PO₄) = (8 : 2). A TiO₂@PPAA-modified gold electrode was activated by incubating the electrodes at 4 °C for 2 h in PBS (pH 7.4) containing 1 mM NHS and 4 mM EDC. The RhB solution was dissolved in ethanol (0.1 mM). Different Cu²⁺ concentrations were diluted from the stock of 1 mM CuSO₄ solution. Interference ions were diluted from the stock of 1 mM MnSO₄, CoSO₄, NiSO₄, AgNO₃, Pb(NO₃)₂, MgCl₂, and ZnCl₂ solutions.

2.3 Characterization

Chemical structures and components of TiO₂, TiO₂@PPAA, and TiO₂@PPAA-RHB were characterized by Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS). FT-IR spectra of the samples were acquired from samples in KBr pellets by using a Nicolet 5700 FT-IR instrument within the range of 400–4000 cm⁻¹. XPS data were acquired using an AXIS HIS 165 spectrometer (Kratos Analytical, Manchester, UK) with a monochromatized Al KR X-ray source (1486.71 eV photons). Field emission scanning electron microscopy images (FE-SEM) were obtained using a JSM-6490LV SEM (Japan). Meanwhile, transmission electron microscopy (TEM) images were acquired using TEM [JEOL-2100 (UHR)]. UV-vis spectra were obtained through a computer-controlled UV-vis spectrometer (TU-1201, Beijing Instrument Company).

2.4 Electrochemical measurements

All electrochemical measurements were performed using a CHI660D electrochemical workstation (Shanghai CH Instrument Company, Shanghai, China) and via a conventional three-electrode system composed of a gold electrode, a Pt slide auxiliary electrode, and a Ag/AgCl reference electrode in a 1 : 1 mixture of 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆], 0.14 M NaCl, and 2 mM KCl. In cyclic voltammetry (CV) experiments, the scan rate was 100 mV s⁻¹. Electrochemical impedance spectra (EIS) was performed in a frequency range of 1 mHz–1 MHz and analyzed using Zview2 software, which is described in the ESI† (Fig. S1). All electrochemical experiments were carried out at room temperature (25 ± 1 °C). Three parallel experiments were conducted for each measurement, and their average values were applied in the present work.

2.5 Construction of the developed electrochemical sensor

Pre-treatment of the bare gold electrode. Prior to the modification with h-TiO₂ microspheres, the gold electrode (1.2 mm in diameter) was polished with 1.0 and 0.3 μm alumina slurry and ultrasonically washed with water, ethanol, and finally water again. Subsequently, the electrode was electrochemically cleaned in 0.1 M H₂SO₄ by potential scanning between −0.2 V and 1.6 V until producible cyclic voltammetry was obtained. Finally, the electrode was rinsed thoroughly with water and dried under nitrogen gas.

Fabrication of the h-TiO₂@PPAA-modified gold electrode. The preparation of hollow TiO₂ microspheres was referred to as reported in the paper of Kang et al.²⁶ Subsequently, 10 mg of h-TiO₂ microspheres were dispersed in 10 mL of ethanol. Approximately 10 μL of TiO₂ (1 mg mL⁻¹) dispersion was dropped onto the surface of the gold electrode and dried in air, followed by further modification with PPAA in a plasma chamber. Plasma modification was performed in a HQ-2 PECVD system manufactured by the Institute of Microelectronics of the Chinese Academy of Sciences, China. The radiofrequency generator was operated at 13.56 MHz. PPAA nanofilm was deposited onto a gold electrode coated with h-TiO₂ microspheres at the plasma input power of 20 W for 1 min in a continuous wave; acrylic acid was used as the monomer. The gas flow rate was fixed at 10 sccm, and the pressure was maintained constant at 0.1 Pa. The h-TiO₂@PPAA-modified electrode was immersed in PBS overnight to remove any unbound monomer molecules or oligomers in the polymeric network.

Preparation of h-TiO₂@PPAA-RhB. The Au electrode modified by the h-TiO₂@PPAA composite was incubated at 4 °C for 2 h in the activated solution containing EDC/NHS. The treated electrodes were rinsed with PBS thoroughly and dried under a N₂ stream. Subsequently, the modified electrode was immersed in a solution of 100 mM rhodamine B for 2 h to form the h-TiO₂@PPAA-RhB-based sensor. Afterward, the developed electrochemical sensor was employed to detect Cu²⁺. The performance of the developed electrochemical sensor for Cu²⁺ detection in practical applications was verified by using the difference of the electron transfer resistance (ΔR_et) value to investigate in the presence of competing metal ions, such as Co²⁺, Ni²⁺, Zn²⁺, Mn²⁺, Pb²⁺, Ag⁺, and Mg²⁺. Meanwhile, practical application was performed by adding Cu²⁺ to the real sample (tap water) using the standard addition method.

3. Results and discussion

The chemical structure of the h-TiO₂@PPAA composite depended on the plasma input power used. The FT-IR spectra of the h-TiO₂@PPAA composite deposited at 20 W exhibited high intensity of the remaining carboxyl groups, which would facilitate RhB binding (Fig. S2, ESI†). Therefore, more Cu²⁺ ions can be coordinated with RhB and determined.

3.1 Surface morphology of the h-TiO₂@PPAA nanocomposite

Fig. 1 shows SEM and TEM analyses on the as-prepared h-TiO₂ microspheres and h-TiO₂@PPAA nanocomposites. As shown in Fig. 1a, SEM images show a spherical morphology with a diameter approximately 300–420 nm. The spherical balls also indicate a porous microstructure. After PPAA deposition onto the surface of h-TiO₂ microspheres, a dense layer of the composite with a rough surface was observed (Fig. 1b and c). Fig. 1d illustrates the typical TEM images of the h-TiO₂@PPAA composite in high magnification. h-TiO₂ microspheres are uniformly covered by the PPAA layer. The wall thickness of the TiO₂@PPAA microcomposite is approximately 25–33 nm, as estimated from Fig. 1e. The high-resolution lattice image shown in Fig. 1f indicates that h-TiO₂ is well crystallized into the single anatase phase with a crystallographic spacing of 0.34 nm, which is the preferred [101] growth direction.²⁷

Fig. 1 SEM images of (a) h-TiO₂ microspheres and (b and c) TiO₂@PPAA composites deposited at 20 W for 1 min, and (d–f) TEM images.

3.2 Chemical structure of the h-TiO₂@PPAA-RhB composite

The modification procedure of the h-TiO₂@PPAA composite with RhB was characterized using XPS, thus providing a detailed variation of chemical structures and components of the composite surface. As shown in Fig. S3 (ESI†), the XPS survey scan spectra of the samples at different steps, including the h-TiO₂@PPAA composite deposited at 20 W for 1 min, activation of the nanocomposite using EDC/NHS, and modification of the nanocomposites with RhB (h-TiO₂@PPAA-RhB), show the presence of C 1s and O 1s. After activation via EDC/NHS and subsequent modification with RhB, an additional N 1s signal was observed in curves b and c. Table S1 (ESI†) summarizes the corresponding atomic% of the samples at different steps. The high-resolution XPS spectra of each element further reveal the variations of the surface functional groups of the h-TiO₂@PPAA composite after being activated through EDC/NHS and modified by RhB. The high-resolution XPS spectrum of C 1s contained in the h-TiO₂@PPAA nanocomposite can be fitted by four peaks at ∼284.8, ∼286.5, ∼288.2, and ∼288.9 eV, which correspond to C–C/C–H, C–O, C=O, and O=C–O groups, respectively (Fig. 2a). After activation, a new peak at the binding energy (BE) of ∼285.32 eV was observed (Fig. 2b), which was caused by the C–N groups. When the carboxyl groups contained in h-TiO₂@PPAA-RhB were transferred to the activated ester groups, the nitrogen elements were introduced. Thus, the possible presence of N–C=O at ∼287.72 and O=C–O at ∼288.42 eV in the activated composites was proven. However, after surface modification by RhB, only three peaks in the high-resolution XPS spectra of C 1s were obtained (Fig. 2c). These peaks include ∼284.71, ∼285.81, and ∼288.21 eV, which are attributed to C–C/C–H, C–N, and N–C=O groups, respectively. The results show that the carboxyl groups of h-TiO₂@PPAA composite are fully changed to the N–C=O groups via RhB modification, thereby leading to the chemical bonding of RhB onto the composite via this feasible method. Furthermore, according to the peak area, the relative content of each functional group can be evaluated. The C–N density in the activated h-TiO₂@PPAA was higher than that in the h-TiO₂@PPAA-RhB because of the introduction of other RhB components. The high N% of activated h-TiO₂@PPAA (16.95%) was also proven. As shown in Fig. 2b, the high-resolution XPS spectrum of N 1s in the activated h-TiO₂@PPAA is fitted into two parts, with BEs of ∼399.16 and ∼401.26 eV assigned with N–H/C–N and O=C–N groups, respectively. The same peaks were observed in the N 1s high-resolution spectrum of the modified composite (Fig. 2c). Apart from the observed C 1s and N 1s in the samples, an extremely weak Ti 2p signal was obtained because the TiO₂ layer was completely covered by the polymerized plasma film (Fig. 2a).

Fig. 2 High-resolution XPS spectra of (a1) C 1s contained in h-TiO₂@PPAA nanocomposites deposited at 20 W for 10 min. High-resolution XPS spectra of (b1) C 1s and (b2) N 1s contained in activated h-TiO₂@PPAA nanocomposites using EDC/NHS. High-resolution XPS spectra of (c1) C 1s and (c2) N 1s contained in h-TiO₂@PPAA-RhB. (a2) Ti 2p core-level XPS spectra of h-TiO₂@PPAA nanocomposites.

3.3 Cu²⁺ detection using the fabricated electrochemical sensor

CV was employed to monitor the fabrication process of the electrochemical sensor using an [Fe(CN)]^3−/4− redox probe. CV provides information about the integrity properties of surface modification on gold surfaces, such as defects, insulating properties, and density of the adsorbed layer. Fig. 3a shows the CVs recorded for the bare gold electrode, h-TiO₂ microspheres-(h-TiO₂/Au), h-TiO₂@PPAA-(h-TiO₂@PPAA/Au), activated h-TiO₂@PPAA-(A-h-TiO₂@PPAA/Au), and h-TiO₂@PPAA-RhB-modified electrode (h-TiO₂@PPAA-RhB/Au). Moreover, h-TiO₂@PPAA-RhB/Au was incubated in Cu²⁺ solution for 0.5 h and recorded by CV. Fig. 3a shows a well-defined quasi-reversible redox cyclic voltammogram with an anodic to cathodic peak ratio of approximately one and peak-to-peak separation ΔE_p of 256 mV for the bare gold electrode (curve a). After dropping the h-TiO₂ nanospheres onto the gold electrode (curve b), the reversibility was maintained the same as that for the bare gold electrode, and the peak current (I_a = 86 μA) was significantly lower than that of the bare gold electrode (I_a = 106 μA). Furthermore, the ΔE_p between the reduction and oxidation waves increased to 314 mV, hence suggesting that the h-TiO₂ nanospheres decreased the electrode conductivity and reduced the electrode transfer rate between the redox probe and the electrode surface.²⁸ For the h-TiO₂@PPAA composite-modified electrode (curve c), the peak current value (I_a = 70.6 μA) decreased substantially with a reduced peak current of 36.5 μA. This result indicates that the coverage of polymerized plasma film evidently prevents the electron transfer and exhibits relatively varying low electrochemical activity.²⁹ However, when the h-TiO₂@PPAA composite was activated using EDC/NHS (curve d), the peak current increased to 70.7 μA. Therefore, activation can promote electron transfer at the interface.³⁰ As the peak current decreased again because of the modification with RhB, the layer with low electrochemical activity was formed between the activated composite and RhB.^7,31 The peak current further decreased to the lowest value when Cu²⁺ ions were coordinated with RhB (h-TiO₂@PPAA-RhB-Cu²⁺/Au), which may be due to the insulation layer produced.²⁶

Fig. 3 (A) CVs and (B) Nyquist plots for impedance measurements corresponding to the bare electrode (curve a) and the composite electrode modified with h-TiO₂ microspheres (curve b), h-TiO₂@PPAA (curve c), A-h-TiO₂@PPAA (curve d), h-TiO₂@PPAA-RhB (curve e), and h-TiO₂@PPAA-RhB-Cu²⁺ (curve f) recorded in a 0.5 M [Fe(CN)]^3−/4− probe in PBS at pH 7.4, over the frequency range of 100–0.1 Hz, bias potential of +0.21 V, and ac amplitude of 10 mV.

EIS is another informative and commonly used technique to investigate biosensors. This technique is applied in this study, in parallel with CV measurement, for the characterization of an h-TiO₂@PPAA electrochemical sensing process (Fig. 3b). The impedance spectra represented in the Nyquist plot are composed of semicircles at high frequencies, which indicate an electron transfer process, and a linear part at low frequencies that correspond to the diffusion-controlled process. Modified Randles equivalent circuit was employed to fit the impedance spectra of the h-TiO₂@PPAA electrochemical sensing (Fig. S1, ESI†). This circuit consists of the electrolyte resistance between the working and reference electrodes, R_s; Warburg impedance associated with the diffusion process of redox probe ions from the bulk solution to the electrode-solution interface, Z_w; electron transfer resistance, R_et; and constant phase element CPE (related to the double-layer capacitance) (Table S2, ESI†).

R _et shows the largest variation during the assembly process of electrochemical sensing. Therefore, R_et is more suitable as a parameter for sensing the interfacial properties of the prepared sensing during the fabrication steps. The bare gold electrode exhibited a small semicircle at high frequency (curve a) of the R_et value (50.21 Ω). After dropping the h-TiO₂ microspheres on the surface of the gold electrode h-TiO₂/Au (curve b), the R_et value of the composite electrode increased to 139.9 Ω; it further increased to 426.8 Ω after the deposition of the PPAA layer (curve c). This observation may be attributed to the combined low conductivity of the plasma polymer, which efficiently promotes the electron transfer resistance, consequently decreasing the diffusion-controlled process.³² The deposited PPAA containing the negatively charged carboxyl groups in aqueous solution may also cause electron transfer retardation between the electrode surface and the negative [Fe(CN)₆]^3−/4− probe. However, the R_et value decreased to 354.2 Ω because of the activation using EDC/NHS.³³ As RhB was applied to modify the activated electrode, the R_et value substantially increased to 797.2 Ω. The coordination of Cu²⁺ with the surface of the h-TiO₂@PPAA-RhB/Au electrode (curve e) enlarged the semicircle diameter, and R_et significantly increased to 1510 Ω because of the electrode surface blockage by the h-TiO₂@PPAA-RhB-Cu²⁺.⁷ The results obtained from EIS measurements agree with the results extracted from CV, thus confirming again the successful coordination of Cu²⁺ on the RhB-modified h-TiO₂@PPAA-RhB/Au electrode and fabrication of Cu²⁺ sensing.

The same electrochemical measurements of Cu²⁺ detection were carried out to investigate the direct adsorption efficiency of Cu²⁺ by utilizing different modified electrodes using the h-TiO₂ microspheres, pristine PPAA nanofilm, and h-TiO₂@PPAA composites (Fig. S4, ESI†). The continuous increase of R_et values was also observed during Cu²⁺ detection (Fig. S5, ESI†). Comparing the three samples, PPAA nanofilm exhibited the poorest electrochemical performance, with the highest R_et value of 1.34 kohm, whereas the hollow TiO₂ microspheres showed a relatively good electrochemical activity. In addition, the results exhibited that Cu²⁺ ions in the water can be adsorbed onto three kinds of materials, in which the adsorption efficiency of Cu²⁺ onto the h-TiO₂@PPAA composite is the most promising. The sorption of Cu(II) by h-TiO₂ occurs through the formation of inner-sphere surface complexes, which originated from the interaction of Cu²⁺ with the hydroxyl groups of the TiO₂ surface.³⁴ For the PPAA nanofilm, the –COOH groups were deprotonated, thereby resulting in the negatively charged ligands (–COO⁻), which easily attract positively charged Cu²⁺.³⁵ The intensity of –COOH contained in PPAA is not extremely high, thus leading to the small amounts of Cu²⁺ on its surface. Consequently, h-TiO₂ cannot only enhance the electrochemical properties of PPAA but also improve the adsorption amounts of Cu²⁺, so that the h-TiO₂@PPAA composite can be used as an electrode material for sensing. However, the developed h-TiO₂@PPAA composite also shows high adsorption efficiency toward other metal ions, such as Pb²⁺, Co²⁺, Mn²⁺, Ni²⁺, Zn²⁺, and Mg²⁺ (Fig. S6, ESI†). The metal ions could possibly coordinate with the carboxylic acid groups and form an inner-sphere complex structure.⁴¹ Hence, modifying the h-TiO₂@PPAA composite for its selectivity toward Cu²⁺ ions is necessary.

3.4 Sensitivity of the fabricated electrochemical sensor toward Cu²⁺ detection

Fig. 4 shows the impedance spectra for the electrochemical sensor obtained from various Cu²⁺ concentrations. The diameter of the semicircle gradually increases in the Nyquist plots as the Cu²⁺ concentration is increased (Fig. 4a). This observation indicates a significant increase in the electron transfer resistance and thus increases the barrier for the electron transfer between the redox probe and the electrode surface. The relative electron transfer resistance values ΔR_et = R_et,(2) − R_et,(1) were used; R_et,(2) and R_et,(1) are the electron transfer resistances of the RhB layer before and after binding with Cu²⁺, respectively.

Fig. 4 (a) EIS Nyquist plots for the detection of different Cu²⁺ concentrations. The concentrations are (from a to e) 0, 0.001, 0.01, 0.1, 1, and 10 nM. (b) The linear fit plots ΔR_et as a function of the logarithm of Cu²⁺ concentration. The error bar represents the standard deviation of three parallel experiments.

As shown in Fig. 4, a linear increase in ΔR_et is obtained with increasing Cu²⁺ concentrations from 0.001 nM to 10 nM by using the regression equation ΔR_et = 0.241 + 0.955 log(C) nM, R² = 0.983. And it has a very low limit of detection (LOD), down to 0.404 pM based on a multiple of 3 the standard deviation of the baseline signal. The samples were dipped in 1 nM Cu²⁺ standard solution four times and the impedance responses were recorded to further investigate the reproducibility of the developed Cu²⁺ sensor. The LOD was not only more superior than the previously reported methods (Table 1) but also lower than the maximum level of Cu in drinking water permitted by the United States Environmental Protection Agency.⁴²

Table 1 Comparison of the analytical performance of the h-TiO₂@PPAA-RhB electrochemical sensor

Sensitive layer	Detection technology	Linear range	LOD	Ref.
Adamantane-modified rhodamine and cyclodextrin-modified Fe3O4@SiO2	UV-vis spectra	0.01–10 mM	5.99 μM	36
P(NIPAM-co-RHBBA)	UV-vis spectra	0–20 μM	0.18 μM	37
Rhodamine B-based fluorescent probe RL	Fluorescence and adsorption spectra	0–8 μM	0.739 nM	38
Graphite-like carbon nitride	Electrogenerated chemiluminescence	2.5–100 nM	0.9 nM	39
Carbon dot-based silica nanoparticles	Fluorescence spectra	0–10 μM	35.2 nM	40
TiO2 modified by fluorescein hydrozin-3,6-diacetic acid	EIS	5 pM–1 μM	4.29 pM	26
h-TiO2@PPAA-RhB	EIS	1 pM–10 nM	0.404 pM	This work

---

3.5 Selectivity

The complexity of the water system presents a significant challenge to the analytical methods for metal ion detection not only in sensitivity but also more importantly in selectivity. Selectivity experiments were carried out by monitoring the R_et value of the h-TiO₂@PPAA-RhB/Au electrode in the Fe(CN)₆^3−/4− solution with the addition of other metal ions. Various metal ions, such as Mn²⁺, Co²⁺, Pb²⁺, Ag⁺, Mg²⁺, Zn²⁺, and Ni²⁺, with 100-fold concentration as that of Cu²⁺ were initially tested. Remarkably, as shown in Fig. 5, no evident electrochemical responses were observed for the other metal ions compared with that obtained for Cu²⁺. Furthermore, these potential metal ion interferences showed negligible effects on the electrochemical signal for Cu²⁺ sensing. The high selectivity of the present biosensor for Cu²⁺ should be ascribed to the specific and stable affinity of RhB toward Cu²⁺. In conclusion, the described protocol based on h-TiO₂@PPAA-RhB could be generalized for other heavy ions by changing the specific fluorescence target for targeted ion testing with high specificity and sensitivity.

Fig. 5 Selectivity of Cu²⁺ sensing assay with 0.001 nM Cu²⁺ and 1 nM of other metal ions.

3.6 Real samples

The practical application of the developed chemical sensing for detecting Cu²⁺ was measured by determining the apparent recovery of the spiked Cu²⁺ ions in tap water and river water. The sample was filtered through 0.2 μm membranes to remove impurities. Subsequently, standard Cu²⁺ solutions with different concentrations (0.001, 0.01, and 0.1 nM) were added to the pretreated water sample. The spiked samples were analyzed separately using the designed electrochemical sensor; the results are shown in Table 2. Apparent recovery values that range from 102% to 130% were obtained, which indicate that the designed sensor is applicable for analyzing Cu²⁺ in real water.

Table 2 Practical application of the developed chemical sensing for Cu²⁺ detection

Real sample	Amount added (nM)	ΔRet (kohm)	Found (nM)	Recovery (%)	RSD%
Tap water	0.001	0.30	0.0012	120	1.3
	0.01	3.10	0.0123	123	1.8
	0.1	3.72	0.106	106	2.5
River water	0.001	0.33	0.0013	130	2.1
	0.01	3.22	0.011	110	2.7
	0.1	3.69	0.098	102	3.1

---

4. Conclusions

In summary, a highly sensitive and selective electrochemical sensing is reported for the quantitative detection of Cu²⁺ based on the rhodamine B-modified h-TiO₂@PPAA composites via the coordination between Cu²⁺ and rhodamine B. The synergistic effect of the presence of TiO₂ and PPAA promoted Cu²⁺ adsorption and subsequently enhanced detection sensitivity. ΔR_et before and after Cu²⁺ addition allowed the detection and quantitative analysis with a low limit of detection of 0.404 pM within the range of 0.001–10 nM (R² = 0.9828). Excellent selectivity toward interfering metal ions, such as Mn²⁺, Co²⁺, Pb²⁺, Ag⁺, Mg²⁺, Zn²⁺, and Ni²⁺, can be obtained. Therefore, this new strategy may offer a new approach for the development of sensitive sensors for Cu²⁺ determination and could find widespread environmental applications. However, since plasma polymers were not very stable in aqueous solution, it is time-consuming for the fabrication of the electrochemical sensors more or less.

Acknowledgements

This work was supported by Program for the National Natural Science Foundation of China (NSFC: Account No. 51173172) and Science and Technology Opening Cooperation Project of Henan Province (Account No. 132106000076).

Notes and references

1. P. Wang, L. Liu, P. Zhou, W. Wu, J. Wu, W. Liu and Y. Tang, Biosens. Bioelectron., 2015, 72, 80–86.
2. J. Wang and Q. Zong, Sens. Actuators, B, 2015, 216, 572–577.
3. S. Hu, J. Song, F. Zhao, X. Meng and G. Wu, Sens. Actuators, B, 2015, 215, 241–248.
4. K. Kandasamy, S. Ganesabaskaran, M. P. Pachamuthu and A. Ramanathan, Spectrochim. Acta, Part A, 2015, 148, 184–188.
5. S. Arain, T. Kazi, H. Afridi, N. Ullah, M. Arain and A. Panhwar, Anal. Methods, 2015, 7, 3431–3437.
6. H. Shoaee, M. Roshdi, N. Khanlarzadeh and A. Beiraghi, Spectrochim. Acta, Part A, 2012, 98, 70–75.
7. M. Kang, D. Peng, Y. Zhang, Y. Yang, L. He, F. Yan, S. Sun, S. Fang, P. Wang and Z. Zhang, New J. Chem., 2015, 39, 3137–3144.
8. Y. Chang, Y. Chai, S. Xie, Y. Yuan, J. Zhang and R. Yuan, Analyst, 2014, 139, 4264–4269.
9. E. Abdel-Halim and S. S. Al-Deyab, React. Funct. Polym., 2014, 75, 1–8.
10. C. Mbareck, Q. T. Nguyen, O. T. Alaoui and D. Barillier, J. Hazard. Mater., 2009, 171, 93–101.
11. G. Zeng, Y. Liu, L. Tang, G. Yang, Y. Pang, Y. Zhang, Y. Zhou, Z. Li, M. Li and M. Lai, Chem. Eng. J., 2015, 259, 153–160.
12. M. Cvek, M. Mrlik, M. Ilcikova, T. Plachy, M. Sedlacik, J. Mosnacek and V. Pavlinek, J. Mater. Chem. C, 2015, 3, 4646–4656.
13. Y. Nakagawa, S. Nakasako, S. Ohta and T. Ito, Carbohydr. Polym., 2015, 117, 43–53.
14. W. Zhang, X. Shi, Y. Zhang, W. Gu, B. Li and Y. Xian, J. Mater. Chem. A, 2013, 1, 1745–1753.
15. P. Rivolo, S. M. Severino, S. Ricciardi, F. Frascella and F. Geobaldo, J. Colloid Interface Sci., 2014, 416, 73–80.
16. F. Michelotti and E. Descrovi, Appl. Phys. Lett., 2011, 99, 231107.
17. G. Gangadhara, U. Maheshwarib and S. Guptac, Nanosci. Nanotechnol., 2012, 2, 141.
18. C. Yu, Y. Wen, X. Qin and J. Zhang, Anal. Methods, 2014, 6, 9825–9830.
19. D. Guo, Z. Dong, C. Luo, W. Zan, S. Yan and X. Yao, RSC Adv., 2014, 4, 5718–5725.
20. H. Zhang, X. Zeng, D. Chen, Y. Guo, W. Jiang, L. Xu and F. Fu, RSC Adv., 2015, 5, 45847–45852.
21. D. Wan, G. Wang, W. Li, K. Chen, L. Lu and Q. Hu, Appl. Surf. Sci., 2015, 349, 988–996.
22. Z. Q. Li, Y. P. Que, L. E. Mo, W. C. Chen, Y. Ding, Y. M. Ma, L. Jiang, L. Hu and S. Dai, ACS Appl. Mater. Interfaces, 2015, 7, 10928–10934.
23. J. Hu and H. J. Shipley, Sci. Total Environ., 2012, 431, 209–220.
24. X. Xie and L. Gao, Curr. Appl. Phys., 2009, 9, S185–S188.
25. S. Yoon and A. Manthiram, J. Phys. Chem. C, 2011, 115, 9410–9416.
26. M. Kang, M. Wang, S. Zhang, X. Dong, L. He, Y. Zhang, D. Guo, P. Wang, S. Fang and Z. Zhang, Electrochim. Acta, 2015, 161, 186–194.
27. Q. Shang, X. Tan, T. Yu, Z. Zhang, Y. Zou and S. Wang, J. Colloid Interface Sci., 2015, 445, 134–444.
28. Z. Yang, Y. Xu, J. Li, Z. Jian, S. Yu, Y. Zhang, X. Hu and D. D. Dionysiou, Microchim. Acta, 2015, 1–8.
29. T. Homma, M. Kondo, T. Kuwahara and M. Shimomura, Eur. Polym. J., 2015, 62, 139–144.
30. Z. Zhang, S. Zhang, L. He, D. Peng, F. Yan, M. Wang, J. Zhao, H. Zhang and S. Fang, Biosens. Bioelectron., 2015, 74, 384–390.
31. J. Austin, I. Harrison and T. Quickenden, J. Phys. Chem., 1986, 90, 1839–1843.
32. C. M. Weikart, Y. Matsuzawa, L. Winterton and H. K. Yasuda, J. Biomed. Mater. Res., 2001, 54, 597–607.
33. R. Pauliukaite, M. E. Ghica, O. Fatibello-Filho and C. M. Brett, Electrochim. Acta, 2010, 55, 6239–6247.
34. C. Ludwig and P. W. Schindler, J. Colloid Interface Sci., 1995, 169, 284–290.
35. N. Li and R. Bai, Ind. Eng. Chem. Res., 2006, 45, 7897–7904.
36. Y. Zhang, W. Wang, Q. Li, Q. Yang, Y. Li and J. Du, Talanta, 2015, 141, 33–40.
37. G. Li, F. Tao, H. Wang, Y. Li and L. Wang, Sens. Actuators, B, 2015, 211, 325–331.
38. Y. Yuan, S. Sun, S. Liu, X. Song and X. Peng, J. Mater. Chem. B, 2015, 3, 5261–5265.
39. C. Cheng, Y. Huang, X. Tian, B. Zheng, Y. Li, H. Yuan, D. Xiao, S. Xie and M. M. Choi, Anal. Chem., 2012, 84, 4754–4759.
40. X. Liu, N. Zhang, T. Bing and D. Shangguan, Anal. Chem., 2014, 86, 2289–2296.
41. W. Li, H. Zhao, P. Teasdale, R. John and S. Zhang, React. Funct. Polym., 2002, 52, 31–41.
42. E. M. Thurman, D. A. Goolsby, M. T. Meyer, M. S. Mills, M. L. Pomes and D. W. Kolpin, Environ. Sci. Technol., 1992, 26, 2440–2447.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5nj02483h

---