Mesoporous SiO2 Nanoparticles: A Unique Platform Enabling Sensitive Detection of Rare Earth Ions with Smartphone Camera
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KeywordsMesoporous silica nanoparticles Rare earth ions Quantitative detection Antenna effect
A protocol for quantitative measurement of Eu3+ ions is developed using mesoporous silica nanoparticles.
A “concentrating effect” of mesoporous silica nanoparticles is responsible for high adsorption capacity (4730 mg g−1) of Eu3+ ions. An “antenna effect” of 1,10-phenanthroline enables enhanced photoemission of adsorbed Eu3+ ions.
The detection limit of Eu3+ ions is 80 nM even with smartphone camera.
The demand for rare earth metals has increased substantially due to their unique properties and applications in hi-tech products ranging from strong magnets, optical lenses, catalysts, aircraft engines, medical devices, and nuclear reactors [1, 2, 3, 4, 5, 6]. However, the market supply is not sufficient because of the limited conservation, poorly developed extraction techniques, and export restrictions . Therefore, exploring new resources of the valuable rare earth elements represents an urgent research direction [8, 9, 10, 11, 12, 13, 14, 15, 16]. The recent efforts include the search and extraction of rare earth metals from waste streams, coals, and industrial residues [8, 10, 16, 17, 18, 19]. Sensitive detection of the low-concentration rare earth elements in these resources is crucial for precisely evaluating the economic value of various resources. The widely used methods include inductively coupled plasma mass spectrometry (ICP-MS), instrumental neutron activation analyses (INAA), X-ray fluorescence (XRF) and Raman spectroscopy. Despite the relatively good reproducibility and accuracy, these methods usually require tedious procedures, specialized detection environments, and expensive equipment. Developing low-cost and field-compatible protocols becomes essential to address the challenges.
In this report, we demonstrate a facile and cost-effective protocol for sensitive detection of rare earth ion with a very low detection limit down to 80 nM, in which Eu3+ ions have been used as a model. The success of this protocol relies on the use of mesoporous silica nanoparticles (MSNs) as a unique type of adsorbents, which can quickly adsorb Eu3+ ions via electrostatic attractions, to efficiently accumulate Eu3+ ions from very diluted solutions onto the MSNs. Due to the high density of mesoscale pores in the MSNs, the concentration of Eu3+ accumulated in the MSNs can be as high as tens of molars (mole L−1), which is denoted as “concentrating effect”. The optical transparency of SiO2 favors a simple approach to record intrinsic narrow-band fluorescent emissions of the rare earth elements. Some trivalent lanthanide ions, e.g., Eu3+ ions, usually have low photoexcitation efficiency because of a low light absorption cross-section and the forbidden transition of 4f orbitals [20, 21]. Forming complexes with sensitizing molecules that can strongly absorb light can transfer energy absorbed by the sensitizing molecules to the rare earth ions, leading to a strong fluorescent emission from the rare earth ions [22, 23, 24]. This sensitizing process refers to an “antenna effect”. For example, using 1,10-phenanthroline (phen) as sensitizing molecules can enhance the red fluorescent emission of Eu3+ ions under UV illumination. The 1,10-phenanthroline molecules efficiently absorb the UV irradiation and the excited molecules transfer energy to Eu3+ ions to promote their strong emission [25, 26]. Coordinating 1,10-phenanthroline molecules with Eu3+ adsorbed on the MSNs forms SiO2/Eu(III)/phen nanoparticles, which combines both the concentrating effect and the antenna effect to significantly increase the detection sensitivity of the red fluorescent emission of Eu3+. The intensified emission can be easily imaged with a smartphone camera to provide quantitative analysis.
3 Experimental Methods
3.1 Synthesis of Mesoporous Silica Nanoparticles
In a typical synthesis, 40 mg of poly(acrylic acid) (PAA, average MW of 1800, Sigma Aldrich) was firstly dissolved in 1.5 mL of aqueous ammonia solution (28–30 wt%, Fisher Scientific). The resulting clear solution was added to 30 mL of ethanol (190 proof, Pharmco-Aaper) in a 100-mL round-bottom flask, while the solution was stirred at 500 revolutions per min (rpm) at room temperature. To this diluted PAA solution was added 750 μL of tetraethyl orthosilicate (TEOS, 98%, Sigma Aldrich) slowly. The TEOS liquid was added as five portions of 150 μL each with an interval of 2 h between two sequential additions to allow sufficient time for growing colloidal MSNs. After the last portion of TEOS was added, the reaction continued for 4 more h. The synthesized MSNs were collected after multiple cycles of centrifugation at 13,000 rpm and washing with deionized (DI) water. The obtained MSN powders were again washed with ethanol and dried in an oven held at 50 °C for 2 h.
3.2 Concentrating Eu3+ on MSNs and Chemical Sensitization
In a typical concentrating process, 10 mL of ethanolic solution of EuCl3 (EuCl3·6H2O, 99.9% trace metal basis, Strem Chemicals, Inc.) with an appropriate concentration was added to 10 mL of ethanolic dispersion containing 2.5 mg of the as-synthesized MSNs. The resulting mixtures containing Eu3+ ions with concentrations of 1 mM, 100, 10, 1 μM, and 100 nM were stirred at 600 rpm at 70 °C for 30 min. Such incubation resulted in that all Eu3+ ions were adsorbed and concentrated in the MSNs, forming SiO2/Eu(III) colloidal particles. The SiO2/Eu(III) colloidal particles were then collected by a centrifugation at 13,000 rpm. To the SiO2/Eu(III) precipitated particles was added 20 mL ethanolic solution of 1 mM 1,10-phenanthroline (Sigma-Aldrich). Sonicating this mixture re-dispersed the SiO2/Eu(III) particles. The dispersion was then continuously stirred at 600 rpm for 30 min at 70 °C. The as-prepared SiO2/Eu(III)/phen nanoparticles were then centrifuged and washed multiple times with ethanol to remove excess phen molecules. The nanoparticles were dried in an oven maintained at 50 °C for 2 h and re-dispersed in 400 μL ethanol for colorimetric analysis. Reducing the dispersion volume from 20 mL to 400 μL resulted in a 50-time increase in the concentration of Eu3+ ions, representing a significant concentrating effect.
Absorption spectra of ethanolic solutions of EuCl3 with various concentrations were collected using a UV–Vis spectrophotometer (Thermo Scientific, Evolution 220). Fluorescence spectra were obtained using a fluorometer (PTI) with an analyzed software (PTI Felix32). Transmission electron microscopy (TEM) images of the MSNs were recorded with a JEOL TEM-1400 microscope operated at 120 kV. Energy-dispersive X-ray (EDX) analysis was carried out using a detector (X-MaxN 50, Oxford Instruments) equipped on a FEI Quanta 450 FEG scanning electron microscope operated at 30 kV. The digital images displaying the fluorescence of the SiO2/Eu(III)/phen nanoparticles were obtained under UV illumination (Kodak UVP Transilluminator TFM-20, excitation wavelength of 302 nm) using a smartphone camera. The images were processed using MATLAB software (R2017b) to analyze the average standard RGB intensity of pixels. Codes provided from MathWorks were used. The red intensities (corresponding to the fluorescence) of samples were then plotted against the concentration of Eu3+ ions of corresponding samples.
4 Results and Discussion
The intrinsic weak optical absorption coefficient of Eu3+ causes the low detection sensitivity of traditional UV–Vis absorption and fluorescence spectroscopies (Fig. S1). To overcome this limitation, the concentrated Eu3+ ions can be sensitized with molecules that can strongly absorb light and transfer energy to the Eu3+ ions. For example, soaking the MSNs with adsorbed Eu3+ ions, which are labeled as SiO2/Eu(III) nanoparticles for convenience, in a solution containing the sensitizer of 1,10-phenanthroline (phen), forming phen-Eu3+ complex in the silica matrix (step ii, Fig. 1). The π–π transition in 1,10-phenanthroline molecules makes them strongly absorb UV light [23, 28]. The photon energy absorbed in 1,10-phenanthroline can then efficiently transfer to Eu3+ ions in the phen-Eu3+ complex to excite the Eu3+ ions , resulting in a red fluorescent emission much stronger than the directly excited Eu3+ ions (Fig. 1). The enhancement in fluorescence of Eu3+ ions originates from the efficient energy transfer from the sensitizer molecules, which behave like antennas to strongly absorb light. This mechanism is called an “antenna effect”. The emitted red light can be directly observed without assistance of instrumentation even for the samples with very low concentration of Eu3+ ions. The synergy of “concentrating effect” and “antenna effect” in the MSNs enables the promise in developing a convenient, low-cost, highly sensitive protocol for analyzing rare earth elements.
The high porosity and high-density negative charges in the MSNs are favorable for efficiently adsorbing Eu3+ cations via electrostatic attraction. Three parameters including adsorption kinetics, the time required to reach adsorption equilibrium, and adsorption capacity, are taken into account to assess the adsorption ability of the MSNs. The adsorption of Eu3+ can reach the equilibrium after the EuCl3 solution is mixed with the MSNs for ~ 15 min at 70 °C. The adsorption capacity is as high as ~ 4730 mg g−1 (Eu3+/MSNs) (Fig. S2). Data fitting reveals that the adsorption of Eu3+ on the MSNs follows a pseudo-second-order kinetics with the initial adsorption rate of 4025 mg (g min)−1 (Fig. S2). The superior adsorption kinetics and adsorption capacity make the MSNs a promising platform for quickly concentrating trace lanthanide ions from dilute solutions.
Soaking the SiO2/Eu(III) nanoparticles in a solution of 1,10-phenanthroline molecules can form a complex with the adsorbed Eu3+ ions. Forming the phen-Eu3+ coordination bonds enables the efficient energy transfer from the 1,10-phenanthroline molecules capable of strongly absorbing UV light, to the Eu3+ ions, which exhibit low optical absorption cross-section and forbidden f–f transitions . Such an energy transfer results in the strong emission of Eu3+ under UV illumination, facilitating a sensitive detection of Eu3+ ions by probing the red fluorescent emission (Fig. S3, top). It is worthy of note that the lack of optical absorption of visible light in SiO2 favors the efficient extraction of the red emission from the SiO2/Eu(III)/phen nanoparticles to the detector. The intensity of fluorescence varies with the amount of the SiO2/Eu(III)/phen nanoparticles assembled on a supporting substrate. Reliable and quantitative analysis requires the precise control over the thickness and uniformity of nanoparticle films, which is nontrivial and very difficult to achieve, in particular for fieldwork.
A solvent of ethanol (190 proof), a solution of 1 mM EuCl3, and a dispersion of the SiO2/Eu(III) nanoparticles exhibit red intensities of 16–24 that mainly originate from weak fluorescence of ethanol–water clusters  and ethanolic EuCl3 solution  (Fig. 4b). With the formation of phen-Eu3+ complex, the red intensities of the dispersion of SiO2/Eu(III)/phen nanoparticles increase significantly, exhibiting the values from 44 to 106 for the samples prepared from the Eu3+ solutions with concentrations from 100 nM to 1 mM (Fig. 4b). The actual intensity of emission from the phen-Eu3+ complex in MSNs is calculated with a subtraction of the maximum intensity of the control samples (i.e., 24), and the corrected intensity, Icorrected, of a sample is presented in Fig. 4b (solid red post). The corrected red intensity exhibits a linear-logarithmic dependence on the concentrations (C, in unit M) of Eu3+ ions in the source solutions, i.e., Icorrected = 15.09 × logC + 132.19 (Fig. 4b). The corresponding detection limit is 82.4 nM according to three times of the maximum deviation of the measurement (3σmax = 17.6). The detection limit can be further improved by increasing the volume of source Eu3+ solutions to concentrate more Eu3+ in the dispersion of the SiO2/Eu(III)/phen nanoparticles. The dynamic detection range is broad in a range of tens of nM to tens of mM.
The combination of concentrating effect, which corresponds to adsorption of Eu3+ to the large-area surfaces of the MSNs, and antenna effect, which originates from the energy transfer of sensitizing 1,10-phenanthroline molecules with strong UV absorption to Eu3+ ions, for the first time, enables the quick detection of rare earth elements without the sophisticated instrumentation. The use of low-cost MSNs as the platform of detection is critical because (1) the high-density mesoscale pores in the MSNs and high-density negative surface charges enable a quick adsorption of large amount of rare earth ions through electrostatic attraction; (2) the large size (e.g., ~ 100 nm) of the MSNs favors the easy collection of the MSNs with adsorbed rare earth ions even through simple centrifugation; and (3) the optical transparency and large refractive index of SiO2 benefit the efficient extraction of light emission from the rare earth ions adsorbed in the pores of the MSNs when optical fluorescence spectroscopy is used for analysis. This protocol is promising for applicable field detection of the strategic rare earth resources, usually with very low concentrations.
The presence of rare earth elements can be directly visualized with eyes or smartphone cameras from the films/dispersions of MSNs with adsorbed rare earth ions and sensitizing molecules. Quantitative analysis can be achieved by processing the digital photographs with MATLAB to determine the concentration of samples according to the calibration curve. The accuracy and sensitivity of this protocol can be further improved by using color-specific cameras, more efficient sensitizing molecules, and MSNs with larger surface areas. Due to the specialty of sensitizing molecules, it is necessary to design and synthesize efficient sensitizers to apply this protocol to sensing various rare earth element ions. Although the fluorescence of Eu3+ is barely influenced by the presence of alkali metal ions (e.g., Na+) and alkaline metal ions (e.g., Ca2+), the fluorescence of Eu3+ detected by the smartphone camera might be significantly influence by some coexisting transition metal ions (e.g., Fe2+ and Cu2+), which induce charge transfer from Eu3+ to them [30, 31]. Therefore, if there is a field sample containing the interfering transition metal ions, an appropriate pretreatment such as ion exchange, solvent extraction, and electromembrane separation [32, 33, 34, 35] is necessary to selectively remove the major coexisting transition metal ions, eliminating their potential influence on the detection of rare earth metal ions.
This work was supported by the start-up and OVPR seed Grant from Temple University. Partial characterizations were performed with the use of TMI (Temple Materials Institute) facilities.
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