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## 1 Introduction

Recently, many publications are concerned with the use of Ru(bpy)32+ electrogenerated chemiluminescence (ECL) for the detection of several types of analytes [1], [2]. Intensive efforts have been devoted to Ru(bpy)32+ solid-state ECL sensing platforms and their design over conventional chemiluminescence (CL). The reported strategies employed thus far mainly focused on utilizing solid supports in uniform dispersion or placing (positioning) them on the surface of the working electrode [3], [4], [5], [6]. Examples of solid-state Ru(bpy)32+ ECL sensors included the utilization of solid supports of various chemical natures, sizes and morphologies, to name a few: SiO2 NPs [7], Sm2O3 NPs [8], graphene oxide [9] and carbon nanospheres [10]. In some cases, the solid support was used as a carrier for the Ru(bpy)32+ molecules; in others, it played a dual role as a carrier and signal enhancer. Some of these solid supports are expensive to obtain and can be laborious to synthesize and functionalize. Generally, the CL reaction mechanism proposed for Ru(bpy)32+ in the presence of a coreactant is as follows [11]:

$\begin{array}{l}{\text{Ru(bpy)}}_{3}{}^{2+}+\text{Oxidant\hspace{0.17em}(Ce}\left(\text{IV}\right)\text{)}\to \\ \text{ }{\text{Ru(bpy)}}_{3}{}^{3+}+{e}^{-}\text{ }\text{Oxidation}\end{array}$

${\text{Ru(bpy)}}_{3}{}^{3+}+\text{Analyte}\to \left[\text{Ru}{\left(\text{bpy}\right)}_{3}{}^{2+}\right]٭\text{ }\text{Excitation}$

$\left[\text{Ru}{\left(\text{bpy}\right)}_{3}{}^{2+}\right]٭\to {\text{Ru(bpy)}}_{3}{}^{2+}+\text{hv}\text{\hspace{0.17em}}\left(610\text{\hspace{0.17em}nm}\right)\text{ }\text{Light\hspace{0.17em}emission}$

Titania (TiO2) NPs are semiconductors that have gained wide attention owing to their low cost of preparation, large yield and high surface area. Additionally, TiO2 NPs are considered to be biologically and chemically inert and insoluble under most conditions, which extended their applications to areas such as in clinical and medical fields [12]. Moreover, they have been used in various applications such as photocatalysis [13], dye sensitization for solar cells [14], degradation of pesticides [15] and hydrogen fuel production [16].

Previously, TiO2 NPs have been used solely for solid-state ECL applications. A limited number of publications used such versatile material in Ru(bpy)32+ ECL detection of various analytes in different types of samples. For example, Ru(bpy)32+-TiO2-perfluorosulfonated ionomer composite films were obtained through the immobilization of Ru(bpy)32+ on TiO2 NPs (size=400 nm) in a multistep reaction by Choi et al. [17] in which sulfonation reaction on the surface of the TiO2 NPs was carried out to form TiO2-SO3H. This was employed to enhance the electrostatic attraction between Ru(bpy)32+ and the surface of the TiO2 NPs for ECL applications. In this work, Nafion was employed to ensure the entrapment of the NPs on the electrode surface and their accessibility to the analytes in the solution. The tested analytes were tripropyl amine, oxalate and erythromycin in human urine samples. In another study conducted by Choi and coworkers [18], a more complex solid-state sensor was developed in a multistep preparation scheme, in which commercial multiwall carbon nanotubes (diameter, 10–20 nm; length, 10–50 mm) were solubilized in TiO2 sol. This mixture was then introduced to an equal volume of Nafion polymer to stabilize the composite on the working electrode surface. The coated electrode was then dipped into a Ru(bpy)32+ solution to immobilize it on the customized composite. The multiwall carbon nanotubes were incorporated to increase the amount of Ru(bpy)32+ immobilized on the composite films and subsequently improve the CL signal for the detection of ethanol. Additionally, Xu and Yu [19] used TiO2 nanotube (TN) arrays rather than NPs as a solid support to physically adsorb Ru(bpy)32+ on their surface for amine detection in water sample. The immobilization of Ru(bpy)32+ was performed by dipping the electrode in 0.1 m Ru(bpy)32+ in phosphate buffer for 1 h at 65°C. The TNs were then coated with a drop of 0.1 m Ru(bpy)32+ (volume was not specified) and dried three more times to ensure the immobilization of Ru(bpy)32+ on the TNs. In this work, no polymer was used to stabilize the TiO2 TNs on the surface of the electrode; the authors prepared their own electrode by growing the Ru-TNs on the surface of the Ti electrode.

To our knowledge, only two publications have dealt with the use of NPs for Ru(bpy)32+ CL reactions, in which the NPs were used as coreactants to improve the CL signal owing to their oxidation/reduction properties and not as solid-state CL sensor [20], [21]. Although the idea of incorporating Ru(bpy)32+ on a solid support for CL applications seem more simplistic and appealing, it has not yet been investigated. Additionally, no data for solid-state CL detection have been reported as opposed to the limited number of publications on solid-state ECL. Despite the fact that CL requires less demanding instrumentation compared with ECL: the use of expensive potentiostat, electrode fouling, polymers and electrolyte solutions are avoided. Additionally, incorporating Ru(bpy)32+ complex into the TiO2 NPs can be achieved in a single step rather than immobilizing it on the NP surface in a multistep and pH-controlled process, as previously been reported.

Thus, we report for the first time the synthesis and characterization of TiO2-Ru(bpy)32+ NPs by incorporating Ru(bpy)32+ into TiO2 sol in a one-pot reaction and their application as a solid-state CL sensor. This hybrid material was characterized to determine its size, morphology, porosity, surface area, Ru(bpy)32+ concentration, UV-Vis absorption and Raman scattering spectra. Subsequently, the TiO2-Ru(bpy)32+ NPs were investigated for their CL activity for the detection of oxalate as a model analyte. The optimized system was then applied to two types of pharmaceutical drugs, imipramine and promazine, to show the versatility of the sensor.

## 2.1 Reagents

Tris(2,2′-bipyridyl)dichlororuthenium(II) hexahydrate, ammonium cerium(IV) sulfate, sulfuric acid (95–97%), titanium tetrachloride (TiCl4, 98%), ethanol (99.9%), sodium oxalate, promazine hydrochloride and imipramine hydrochloride were purchased from Sigma-Aldrich Chemie GmbH, Schnelldorf, Germany. Double-distilled water was used throughout all experiments. All chemicals were used as received without further purification.

## 2.2 Apparatus and equipment

The morphological properties of the TiO2-Ru(bpy)32+ NPs were determined using transmission electron microscopy (TEM) with a JEOL JEM 1230 (JEOL Ltd., Japan) operating at 120 kV. The optical properties of the samples were determined using UV-Vis spectra and were recorded using Agilent Cary 5000 UV-Vis spectrometer. Raman spectra of the NP samples were measured using Renishaw inVia confocal Raman spectrometer, with an excitation wavelength of 514 nm line of an argon ion laser and exposure time of 10 s and a grating of 2400 l/mm connected to a CCD camera detector. N2 adsorption-desorption isotherms were determined at −195°C using a model Gemini VII, ASAP 2020 automatic Micromeritics sorptometers (USA), equipped with an out gassing platform. Inductively coupled plasma-optical emission spectrometer (ICP-OES), Perkin Elmer, Optima 7300 DV, was used to measure the amounts of ruthenium and titanium in the TiO2-Ru(bpy)32+ NPs.

For CL detection, H11890-01 Photon Counting Head RoHS Conform (Hamamatsu Photonics UK Limited, Hertfordshire, UK) was used to collect the CL signal. The analyte was introduced into the reaction vial using a syringe pump (Graseby 3200, Worcs, UK).

## 2.3.1 Preparation of TiO2-Ru(bpy)32+ NPs

The TiO2 NPs were prepared as described in our previous work [13] with a minor modification. To the ethanol and water mixture, a certain amount of Ru(bpy)32+ complex was added and allowed to dissolve at room temperature before adding the titanium precursor (TiCl4). The amounts of Ru(bpy)32+ were 0.692 and 0.069 g, which corresponded to 1:10 and 1:100 Ti:Ru molar ratio and referred to in the text as diluted and concentrated, respectively. The formed NPs were washed repeatedly with ethanol and dried overnight under vacuum at 50°C.

## 2.3.2 Sample preparation for ICP-OES

A 0.1 g of TiO2-Ru(bpy)32+ NPs was digested using block digestion in acidic solution (mixture of HNO3:HCl:H2O2). The digestate was subsequently analyzed using ICP-OES.

## 2.3.3 CL of TiO2-Ru(bpy)32+ NPs

The setup consisted of a homemade wooden box in which the photon counting head was placed and isolated from external light source. The CL reaction was performed in a glass vial that contained a certain amount of TiO2-Ru(bpy)32+ NPs, which was then mixed with 1 ml of 5 mm ammonium cerium(IV) sulfate in 0.25 m sulfuric acid solution and placed directly on top of the photon counting head using a homemade plastic holder. The oxidant was added to the TiO2-Ru(bpy)32+ NPs using a micropipette and vortexed to form a green dispersion. The analyte was introduced to the mix using a syringe pump to control the addition flow rate as illustrated in Figure 1.

Figure 1:

A schematic representation of the experiemental setup and reaction scheme for the TiO2-Ru(bpy)32+ solid-state CL sensor.

## 3.1.1 Size and morphology

The surface morphology of plain TiO2, diluted and concentrated TiO2-Ru(bpy)32+ NPs was determined by TEM (Figure 2A–C). TEM images of the synthesized TiO2 NPs showed small size particles (~5 nm) of irregular semispherical shape, which remained almost unchanged in the hybrid TiO2-Ru(bpy)32+ NPs. The Ru(bpy)32+ complex was shown to be homogeneously dispersed on TiO2 NPs.

Figure 2:

TEM images of the (A) plain TiO2 and (B) diluted and (C) concentrated TiO2-Ru(bpy)32+ NPs.

## 3.1.2 N2 isotherms

The N2 isotherms and their pore size vs. pore volume data were obtained for all three samples: plain TiO2 and diluted and concentrated TiO2-Ru(bpy)32+ NPs (Figure 3), while the pore size, pore volume and surface area for the same samples are displayed in Table 1. N2 isotherms obtained for all samples can be ascribed as type IV, and the hysteresis loop can be classified in the middle between H2 and H4 types, indicating a mesoporous structure with slit-shaped pores.

Figure 3:

N2 adsorption-desorption isotherms of the plain TiO2, diluted and concentrated TiO2-Ru(bpy)32+ NPs.

The inset is the pore size distributions for the samples.

Table 1:

Barrett-Joyner-Halenda average pore diameter (Dp), volume (Vp) and specific surface area (SBET) for the plain TiO2 and diluted and concentrated TiO2-Ru(bpy)32+ NPs.

Furthermore, N2 sorpometry measurements shown in Table 1 revealed that the incorporation and the concentration of Ru(bpy)32+ with the TiO2 NP surface had a detrimental effect on the specific surface area. It was observed that as the amount of Ru(bpy)32+ increased, the surface area and pore volume of the hybrid NPs decreased. This could be attributed to the amount of TiO2 present in both the diluted and concentrated TiO2-Ru(bpy)32+ NPs, which is less compared with the pure TiO2 NPs (also supported by the ICP data, discussed later). As the TiO2 NPs only contributed to the surface area and porosity of the TiO2-Ru(bpy)32+ NPs, lowering the amount of TiO2 in the NPs caused the reduction in the surface area and pore volume.

## 3.1.3 UV-Vis spectroscopy

The absorption spectra of the plain TiO2, diluted TiO2-Ru(bpy)32+ and concentrated TiO2-Ru(bpy)32+ NPs were measured using UV-Vis spectrometer in the range of 200–800 nm, as shown in Figure 4A.

Figure 4:

(A) UV-Vis absorption spectra of the plain TiO2, diluted TiO2-Ru(bpy)32+ and concentrated TiO2-Ru(bpy)32+ NPs. (B) A plot of (αhν)2 vs. for the plain TiO2, diluted TiO2-Ru(bpy)32+ and concentrated TiO2-Ru(bpy)32+ NPs.

UV-Vis spectrum of the plain TiO2 NPs exhibited a sharp peak at 360 nm owing to the transitions of electrons from valence band to conductance band. However, the UV-Vis spectra of the diluted and concentrated TiO2-Ru(bpy)32+ NPs showed two absorption peaks: the characteristic absorption band of TiO2 NPs at 360 nm, in addition to the band at 450 nm, which is ascribed to the Ru(bpy)32+ complex metal-to-ligand charge transfer (MLCT) [22]. This indicated the successful incorporation of Ru(bpy)32+ complex in the TiO2 NPs and formation of the hybrid material.

Additionally, almost no difference in λmax=450 nm between the TiO2-Ru(bpy)32+ NPs and liquid phase Ru(bpy)32+ (data not shown) can be observed, which is an indication of the negligible surface association between Ru(bpy)32+ molecules and TiO2 NPs [23]. Quantitatively, the peak at 450 nm for diluted and concentrated TiO2-Ru(bpy)32+ NPs showed a difference in peak intensity, which can be related to the Ru(bpy)32+ concentration (or loading capacity) in the TiO2-Ru(bpy)32+ NPs. A higher intensity was observed with the concentrated TiO2-Ru(bpy)32+ NPs compared with the diluted ones.

From the UV-Vis spectra, a red shift was observed for the TiO2 NPs in the presence of the Ru(bpy)32+. The band gap values of the plain TiO2, diluted TiO2-Ru(bpy)32+ and concentrated TiO2-Ru(bpy)32+ NPs were calculated according to Tauc method using the following equation:

$\left(\alpha h\nu \right)1/m=k\left(h\nu -{E}_{\text{g}}\right)$

where Eg is the optical band gap energy, k is a constant and m=1/2 for a direct energy band gap. For this purpose, a plot of (αhν)2 vs. was constructed, and the linear portion of the plot was extrapolated to the ordinate as shown in Figure 4B. The values were 3.53, 2.53 and 2.57 eV for TiO2, diluted TiO2-Ru(bpy)32+ and concentrated TiO2-Ru(bpy)32+ NPs, respectively.

## 3.1.4 Raman spectroscopy

Figure 5 shows the Raman spectra obtained for the plain TiO2, diluted and concentrated TiO2-Ru(bpy)32+ NPs. The Raman spectrum of the plain TiO2 NPs was dominated by four active modes detected at 150, 402, 512 and 636 cm−1, which can be assigned to the vibrational modes of the pure anatase phase of TiO2 NPs [24].

Figure 5:

Raman spectra of the plain TiO2, diluted TiO2-Ru(bpy)32+ and concentrated TiO2-Ru(bpy)32+ NPs.

On the other hand, the Raman spectrum of the diluted TiO2-Ru(bpy)32+ NPs showed additional peaks at 1021, 1168 and 1353 cm−1, which can be attributed to the presence of Ru(bpy)32+ in the TiO2 NPs [25]. Furthermore, a stronger enhancement of 1050, 1180, 1285 and 1353 cm−1 vibration bands and a decrease in band intensity for 150, 402, 512 and 636 cm−1 were observed for the concentrated TiO2-Ru(bpy)32+ NPs, owing to the presence of larger amounts of Ru(bpy)32+ in the TiO2 NPs.

## 3.1.5 ICP-OES

For the diluted and concentrated TiO2-Ru(bpy)32+ NPs, the amount of Ru(bpy)32+ incorporated into TiO2 NPs was determined by using ICP-OES. For the diluted TiO2-Ru(bpy)32+ NPs (1:100 Ti:Ru molar ratio), the molar concentrations of Ti and Ru metals in the TiO2-Ru(bpy)32+ NPs were 3.75×10−3 and 3.42×10−5m, respectively. Whereas for the concentrated TiO2-Ru(bpy)32+ NPs (1:10 Ti:Ru molar ratio), the values were 1.8×10−3 and 3×10−4m, respectively. This was an indication of incorporation of the Ru(bpy)32+ into the TiO2 NPs. Moreover, as the loaded amount of Ru in TiO2 NPs increased, the concentration of Ru in TiO2 hybrid is also increased.

## 3.2.1 Control experiment

As a proof of concept, CL was investigated for the plain TiO2 NPs and the diluted (3×10−4m) and concentrated (3.42×10−5m) solutions of Ru(bpy)32+ in 0.25 m sulfuric acid. In addition, the synthesized diluted and concentrated hybrid TiO2-Ru(bpy)32+ NPs were also tested. Oxalate was used as a model analyte owing to its low cost and intense CL response (Figure 6).

Figure 6:

The CL signal detected using 0.5 mm Ce(IV) in 0.25 m sulfuric acid as oxidant and 1 mm oxalate as the analyte: 1, plain TiO2 NPs; 2, concentrated Ru(bpy)32+ aqueous solution; 3, diluted Ru(bpy)32+ aqueous solution; 4, concentrated TiO2-Ru(bpy)32+ NPs; 5, diluted TiO2-Ru(bpy)32+ NPs.

The results showed that the plain TiO2 NPs did not exhibit any CL signal after adding the Ce(IV) and oxalate. On the other hand, in the presence of Ru(bpy)32+ solution, the signal was affected by the concentration of Ru(bpy)32+. The higher concentration of Ru(bpy)32+ produced a lower CL signal compared with the lower concentration of Ru(bpy)32+, which could be attributed to the low concentration of the oxidant used. In the case of both diluted and concentrated TiO2-Ru(bpy)32+ NPs, the CL signals were comparable, and the diluted concentration was slightly higher than the concentrated one, which was consistent with the liquid phase results.

Interestingly, the TiO2-Ru(bpy)32+ NPs produced signals higher than the aqueous system, which was due to the large surface area of TiO2-Ru(bpy)32+ NPs, providing better interaction between the Ru(bpy)32+ and both the oxidant and oxalate.

For further experiments, the diluted TiO2-Ru(bpy)32+ NPs were utilized because they contained smaller amounts of Ru(bpy)32+.

## 3.2.2 Effect of the flow rate

To explore the optimum experimental conditions of the constructed solid-state CL sensor, the flow rate at which the analyte was introduced into the TiO2-Ru(bpy)33+ NPs was investigated. The CL signal was measured at flow rates of 100, 125, 150, 175 and 200 ml/h, and the results are shown in Figure 7.

Figure 7:

The effect of analyte (oxalate) flow rate (ml/h) on CL signal using diluted TiO2-Ru(bpy)33+ NPs.

As can be seen from the figure, as the flow rate increased, the CL signal also increased, which is consistent with previously reported data [26]. The maximum CL signal was obtained using a 200-ml/h flow rate. Unfortunately, because of the flow rate restriction from the syringe pump, higher flow rates could not be explored. For the rest of the experiments, analyte flow rate addition of 200 ml/h was used.

## 3.2.3 Effect of the amount of TiO2-Ru(bpy)32+ NPs

Considering the amount of TiO2-Ru(bpy)32+ NPs on the CL signal is a vital parameter; thus, the effect of the amount (in mg) used in each experiment was studied. Quantities of 10, 20, 30, 40 and 50 mg of the TiO2-Ru(bpy)32+ NPs were tested using 0.5 mm Ce(IV) and oxalate solution (Figure 8A and B). Thus, the optimum amount of the TiO2-Ru(bpy)32+ NPs was 20 mg, which is considered suitable as higher amounts are undesirable for low-cost and efficient systems.

Figure 8:

(A) The individual CL signal vs. time using different amounts of TiO2-Ru(bpy)32+ NPs. (B) The effect of the amount of TiO2-Ru(bpy)32+ NPs (mg) using oxalate as an analyte at a flow rate of 200 ml/h.

## 3.2.4 Calibration using oxalate

Oxalate detection is very useful in areas of food chemistry [27] and clinical analysis [28], [29]. For example, monitoring the amount of oxalate in blood or urine can help physicians diagnose certain illnesses such as kidney diseases [30]. One way to monitor the concentration of oxalate is using conventional Ru(bpy)32+ CL or ECL [31], [32]. Thus, we used oxalate as one of the analytes to test the TiO2-Ru(bpy)32+ NP CL system; the reaction mechanism can be found elsewhere [33]. Besides the optimization reported above, a calibration curve was constructed for oxalate as shown in Figure 9.

Figure 9:

Calibration curve obtained for sodium oxalate using diluted TiO2-Ru(bpy)32+ NPs.

Conditions: 0.5 mm Ce(IV) and 200 ml/h flow rate. The oxalate structure is also shown.

The linear range obtained for oxalate was between 10 and 100 pm using our TiO2-Ru(bpy)32+ NP CL sensor as illustrated in Figure 9, with LOD of 1 pm. The values of the linear detection range and LOD for oxalate reported in the literature using different types of CL/ECL systems are shown in Table 2. Compared with the tabulated results, the TiO2-Ru(bpy)32+ NP solid-state sensor has improved the linear range and lowered the LOD of oxalate compared with that of previous work that reported using CL and ECL.

Table 2:

Linear range and LOD of oxalate determined using different types of CL sensors found in the literature.

## 3.2.5 Testing the solid-state CL sensor using pharmaceutical drugs

The TiO2-Ru(bpy)32+ NP sensor was further exploited to detect two pharmaceutical drugs, namely, imipramine and promazine. Imipramine is an example of tricyclic antidepressant drugs, which are commonly prescribed for the treatment of depression and other psychiatric disorders [39], whereas promazine is used as an antipsychotic and an antiemetic agent [40]. A calibration curve for both imipramine and promazine was plotted using the optimized conditions as shown in Figure 10A and B.

Figure 10:

Calibration curve for (A) imipramine drug and (B) promazine drug.

Conditions: 0.5 mm Ce(IV) and 200 ml/h flow rate. The structures of the drugs are also shown.

The linear range obtained for imipramine was 1–100 pm using TiO2-Ru(bpy)32+ NPs. Compared with a recently published data by Sedaghati and coworkers [41], which relied on a quite complicated ECL system, rather than CL, based on Ru(bpy)32+-palladium NPs doped carbon ionic liquid electrode, they reported linearity and LOD similar to our TiO2-Ru(bpy)32+ NPs sensor. Additionally, promazine showed a linear range similar to that obtained for imipramine with LOD of 0.5 pm. Previously reported data for promazine involved the use of Ru(bpy)32+-Ce(IV) in acidic media. The reported linear range was 0.4–30.0 μg/ml and LOD of 0.1 μg/ml [42].

## 4 Conclusion

A simple, rapid and low-cost detection system based on solid-state CL as opposed to ECL was lucratively developed. The TiO2-Ru(bpy)32+ NPs was prepared, characterized and used for CL detection of oxalate, imipramine and promazine. The detection system operated well for the detection of the analytes without the need for polymers for temporal and spatial control or expensive electrical components to help generate and detect the CL signal. The developed system was optimized and can be applied to various types of analytes with improved LOD than previously reported data in the literature. This system could be transferred to a microfluidic platform for better efficiencies and applied to real life samples using minute amounts of reagents and analytes.

## Acknowledgments

The authors gratefully acknowledge the support of the Kuwait University Research Administration for funding project numbers SC 02/17 and SC 06/16. Special thanks to RSPU Facilities (GS 01/01, GS 02/01, GS 03/01 and NUERS (srul 01/13)).

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