A long-wave optical pH sensor based on red upconversion luminescence of NaGdF₄ nanotubes

Abstract

A long-wave optical pH sensor based on intense red upconversion (UC) luminescence of NaGdF₄ nanotubes and bromothymol blue was explored. By detecting the UC spectra of NaGdF₄ nanotubes in aqueous solution, the sensing system could work on the pH measurement. The intensities of the red emission were linear with pH value changing from 6 to 8.

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Upconversion is an anti-Stokes process that involves the conversion of lower-energy photon radiation (usually near-infrared) into higher-energy output emissions. The unique luminescence properties of lanthanide-doped upconversion nanoparticles (UCNPs) have attracted great attention in various research fields, owing to their fundamental scientific importance and potential applications in lasers, optical data storage, biological imaging, analytical sensors for heavy metals and pH value, photodynamic therapy, and solar energy conversion.^1–7

The optical pH sensors respond to pH value with change in optical properties. An ideal sensor shows a clear trend towards operating at wavelengths higher than 500 nm.^8,9 It is because most biological matter shows strong absorption and background luminescence in the UV and visible light below 500 nm to some degree. Lanthanide-doped UCNPs possess attractive features including low autofluorescence, long-wavelength light emitting at 980 nm, low cytotoxicity, less scattering and absorption, deep penetration in biological samples and normally long-wavelength light emissions (longer than 500 nm). This makes lanthanide-doped UNCPs good alternatives for pH sensors in biological applications.^10–17 As mentioned above, UCNPs could become ideal pH sensors by properly designing the UC emissions to get proper long-wavelength emissions. Sun and co-workers designed the first optical pH sensor based on the phenomenon of upconversion luminescence and a hydrogel matrix.¹⁸ They also indicated that the pH sensor may act as a transducer in sensors for acidic gases or basic gases, and in enzymatic biosensors where protons are consumed or produced.

Herein the successful synthesis of lanthanide-doped NaGdF₄ nanotubes by a hydrothermal and subsequent ion-exchange route were presented. NaGdF₄ nanotubes could be easily dispersed in water and exhibit intense red upconversion luminescence centered at 650 nm. When combined with bromothymol blue (BTB), the system can easily perform pH measurements by using a 980 nm NIR laser as the excitation source and detecting UC luminescence. The luminescent intensity of lanthanide-doped NaGdF₄ nanotubes is linear with the respect to pH values ranging from 6 to 8.

NaGdF₄ nanotubes were prepared by hydrothermal and in situ ion exchange processes according to the reference (see ESI†).^19,20 The typical morphologies of the as-synthesized precursors are shown in Fig. 1. A number of uniform nanotubes with diameter around 200 nm and length about 1.2 μm could be clearly observed. The hollow nature of the 1D Gd(OH)₃ nanostructure was observed by TEM investigations. And the XRD patterns of the nanotubes indicated that the as-prepared sample was hexagonal Gd(OH)₃ (JCPDS: 83-2037) (Fig. S1†).

Fig. 1 SEM (A and B), TEM (C), and high-resolution TEM (D) image of as-prepared Gd(OH)₃ nanotubes.

The influences of experimental parameters on the final morphology of the Gd(OH)₃ samples were studied. From the SEM images, it was determined that the Gd³⁺ concentration influences the diameter of the Gd(OH)₃ nanotubes. High Gd³⁺concentration leads to a large diameter and well-dispersed nanotubes (Fig. S2†). When the added amount of NH₄⁺ is less than 0.3 mL, no tubular shape sample can be obtained (Fig. S3A†). The tubular-shaped sample could be obtained by adding more than 0.6 mL NH₄⁺. Higher concentrations of NH₄⁺ lead to well-dispersed and smaller-diameter nanotubes (Fig. S3†).

NaGdF₄ nanotubes were obtained by an in situ ion exchange process by using NaF and HF. The XRD patterns of the sample after the ion exchange indicated a hexagonal phase for NaGdF₄ (JCPDS: 27-0699) (Fig. S1B†). The characteristic morphology of the nanotubes remained unchanged after the ion exchange process (Fig. 2B). However, after the ion exchange, the porous structure appeared on the tube wall (Fig. 2D). In order to enhance the UC luminescence, the as-prepared NaGdF₄ nanotubes were calcinated at 400 °C for 4 h. Importantly, the morphology of the nanotubes remains almost the same due to higher activation energies needed for the collapse of these structures (Fig. 2C).

Fig. 2 SEM images of as-prepared samples: (A) before ion exchange, (B) after ion exchange, (C) after calcinations, and (D) TEM image of NaGdF₄ nanotubes.

The UC PL properties of 20%Yb³⁺/2%Er³⁺ co-doped NaGdF₄ nanotubes after calcination were investigated. The typical UC PL spectrum of NaGdF₄:20%Yb³⁺/2%Er³⁺ nanotubes excited by using a 980 nm continuous-wave laser diode is shown in Fig. S4.† The NaGdF₄ nanotubes showed intense red emissions centered at 650 nm (⁴F_9/2 → ⁴I_15/2) and very weak green emissions at 550 nm (⁴S_3/2 → ⁴I_15/2) and 520 nm (²H_11/2 → ⁴I_15/2). It is known that blood has a very narrow optical window at around 660–700 nm.¹⁸ UC spectra of NaGdF₄ nanotubes fall precisely into this region. Thus, the as-prepared NaGdF₄ nanotubes may serve as an ideal sensor for blood in room air.

BTB is a non-toxic pH probe that undergoes distinct colour changes as pH changes, and covers the physiological pH range. Its absorption spectra in the buffer solution were investigated for pH values ranging from 3 to 8 (see Fig. 3). The pK_a value of BTB in water at 25 °C has been reported to be 6.82.²¹ The colour of BTB changes with pH from yellow (pH < 6) to green (pH = 6–7) and then to blue (at pH ≥ 8). Depending on whether BTB is present in its (blue) base form or in its (yellow) weak acidic form, the probe was expected to exert an insignificant inner filter effect on the red emission of nanotubes.

Fig. 3 Absorption spectra of BTB at pH value from 3 to 8; inset shows linear relationship of 600 nm absorption with an accurate pH value from 6 to 8.

The pH-sensing experiments were performed with BTB and NaGdF₄ nanotubes in water. By changing the pH of the system, the intensities of the system's red emissions (excited by the 980 nm laser) vary with pH, even though the nanotubes exhibited an emission that is independent of pH value. More importantly, the intensities of the red emissions were linear with the change of pH value from 6 to 8 (Fig. 4). Thus, it was indicated that this system has the potential to accurately determine the local pH value of blood in room air. In particular, the pH value around cancer cells is about 6.8, which is lower than normal human cells (7.0–7.4).^22,23 Thus, this system may be used in the detection of cancer cells in future. Moreover, the system may be used for accurate determination of local pH values inside cells.

Fig. 4 UC PL spectra of NaGdF₄:20%Yb³⁺/2%Er³⁺ nanotubes in buffer solutions at pH values from 6 to 8; inset shows linear relationship of red emissions with accurate pH values from 6 to 8.

In conclusion, UC PL NaGdF₄ nanotubes were synthesized by a hydrothermal and ion-exchange process. The influence of the experimental parameters on sample morphology was investigated. When combined with BTB, NaGdF₄ nanotubes can work on pH measurement by detecting the UC PL spectra in aqueous solutions. More importantly, the intensities of the red emission are linear with the change in pH value from 6 to 8. This system has great potential for pH measurement of blood in room air, especially for cancer detection.

Acknowledgements

The authors are grateful for financial aid from the National Natural Science Foundation of China (grant nos 21371165, 51372242, 91122030, and 21210001), National Natural Science Foundation for Creative Research Group (grant no. 21221061), and Jilin Province Youth Foundation (201201008).

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Footnote

† Electronic supplementary information (ESI) available: Detailed experimental procedures; XRD patterns of as-prepared samples; SEM images of samples, UC PL and absorption spectra of samples. See DOI: 10.1039/c4ra09686j

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