A SiO2-coumarin nanohybrid was investigated for its Cu(II) sensing performance in aqueous media, and in comparison with the Cu(II)-selective coumarin used alone. Fluorescence of both coumarin itself and the nanohybrid, λex/λem 435/481 nm, was selectively quenched by Cu(II) when tested against a range of multivalent cations. The nanohybrid had enhanced Cu(II) sensing properties when compared to the coumarin including (i) improved limit of detection from μM-level (0.48 μM) of Cu(II) using coumarin alone to nM-level (0.033 μM) and (ii) an extended linear detection range of 0.033–260 μM (0.0005–4.1 mg/mL) Cu(II) compared to 0.48–55 μM for the coumarin itself. The lower limit of detection and extended range were achieved with a smaller amount of coumarin and no traces of organic solvents used to help coumarin dissolution. Characterization suggested that under applied test conditions at pH = 5, SiO2 nanoparticles with negative surface charges adsorbed coumarin and then (when present) Cu(II) ions. The SiO2-coumarin nanohybrid was then applied for the determination of Cu(II) levels in aqueous soil extracts reaching over 94% recovery rates when used against the standard soil analysis method by inductively coupled plasma mass spectrometry (ICP-MS).
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Introduction
Copper is an essential trace element that plays a significant role in biological and environmental applications. In the human body, Cu(II)- and Cu(II)-dependent enzymes are involved in critical biological processes to ensure normal cellular function (Grasso et al. 2013; Domaille et al. 2010). However, excessive copper intake can lead to imbalance and harmful effects in living organisms (Hao et al. 2015; Zhang et al. 2014). In animals and humans, Cu(II) has been shown to have a role in the development of neurodegenerative disorders such as Menkes, Wilson’s, and Alzheimer’s diseases, and influence the development of certain cancers (Ge et al. 2021). Copper exposure can occur via contamination of soil and drinking water originating mostly from human activities, such as corrosion (Jakeria et al. 2021; Tang et al. 2021), mining, or lackadaisical waste management practices. Thus, easy-to-use, affordable, and sustainable sensing methods for Cu(II) are highly sought after in clinical, food/water, and environmental industries (Ji et al. 2017; Zhu et al. 2012). In addition, the allowable Cu(II) is highly regulated in many environmental media, such as in soil (e.g. 100 mg/kg in Australia) (He et al. 2015) and drinking water (e.g. 2.0 mg/mL in Australia) (Tang et al. 2021; Seeley et al. 2013).
Among existing sensing techniques, optical sensing using colorimetric or fluorescent molecular probes is readily available and non-expensive tool for the analysis of liquid samples and dissolved Cu(II) levels. An often employed sub-class of fluorescent chemosensors for Cu(II) (Zhang et al. 2014; Ramdass et al. 2017; Sivaraman et al. 2018) sensing are coumarin derivatives (Kraljević et al. 2016; Seo et al. 2011) due to their sufficient photostability, tuneable sensitivity, and selectivity (Geißler et al. 2005; Li et al. 2019). For example, hydrazine derivative 1 was developed as a “turn-on” fluorescent probe for the detection of Cu(II) in aqueous acetonitrile with a limit of detection (LOD) of 0.058 μM (Table 1) (Zhang et al. 2019). In comparison, the coumarin-N-acylhydrazone derivative 2 was utilized for both colorimetric and fluorometric detection of Cu(II) in CH3CN/H2O (v/v, 9/1) with a LOD of 8 μM (Table 1) (Li et al. 2019). While there are a large number of fluorescent probes reported for the sensing of particular metal ions in solution, and despite their satisfactory selectivity, they are often limited by insufficient water solubility reducing their ease of use and applicability for environmentally friendly, on-site sensing (Huang et al. 2014; Wang et al. 2016) Another limiting factor is their inability to selectively distinguish the copper cations from a mixture containing multiple metal cations with similar electronic properties, such as in the case of soil extracts.
Table 1
Examples of fluorescent Cu(II) sensors and their reported excitation and emission wavelengths, and limits of detection in given solvents
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One way to overcome mentioned shortfalls is to use nanomaterials that can be surface modified with molecular probes, and that facilitates the dispersion of analyte-selective dyes in aqueous solutions while preserving the sensing performance, especially selectivity (Lin et al. 2006). Such approaches often result in improvements in the limit of detection (LOD) due to the changes in surface to mass ratio of the hybrid sensing materials (Meiling and Guowen 2017) Among others, SiO2 nanoparticles (SiO2 NPs) are water dispersible and non-toxic with well-established, facile synthesis methods via, for example, the the Stöber method (Stöber et al. 1968). A previously reported combination using negatively charged SiO2 NPs and positively charged molecular probe 4 (Table 1) was demonstrated for its use as a metal ion sensor in aqueous media (Peng et al. 2018). The authors discussed that electrostatic interactions between the dye 4 and the SiO2 NP surface resulted in a locally increased concentration of 4 while an enhancement of the fluorescence intensity was recorded. For Cu(II) in tap water, use of 0.25 mg/mL SiO2-4 was delivering a sensing range of 11.2–100 nM and an LOD of 0.2 nM, an improvement from the LOD of 10 μM when 4 was used alone (Peng et al. 2018). A fluorescent, covalently appended SiO2-naphthalin sensor was also reported for Hg(II) detection with a sensing range of 0.1–1 μM and LOD of 6.8 nM in water between pH 4.5 and 8 (He et al. 2009).
The current study aimed to improve the aqueous dispersibility and Cu(II) sensing performance of previously reported coumarin 3 (Qian et al. 2019) by coupling to non-toxic SiO2 NPs to form SiO2-3 nanohybrid. The aim of this work is for on-site, facile, low-cost measurements of water and soil quality that can be conducted by non-trained personnel.
Synthesis of coumarin 3, SiO2 NPs, and the SiO2-3 nanohybrid is described. Then relevant optical, physical, and chemical properties are discussed and compared for each material in aqueous media and in the absence and presence of Cu(II) and other metal ions. While the interaction of Cu(II) with coumarin 3 is well studied, here the focus is given to its interaction with SiO2 NPs and Cu(II) to understand how these affect the observed sensing properties. Finally, the SiO2-3 nanohybrid was tested on aqueous extracts of soil samples and its Cu(II) sensing performance was validated against the industry standard, inductively coupled plasma mass spectrometry (ICP-MS) method.
Experiment
Materials and instrumentations
Ammonia solution (30%) was purchased from Chem-supply. Tetraethyl orthosilicate (TEOS, 99%), diethyl malonate (99%), piperidine (99.5%), 4-(diethylamino)-2-hydroxybenzaldehyde (AR), and trifluoroacetic acid (99%) were purchased from Sigma-Aldrich (USA). 2-(Aminomethyl) pyridine (99%) was obtained from Acros Organics (Belgium, USA). All chemicals were used as received without further purification. Benzotriazol-1-(yloxy)tripyrrolidinophosphonium hexafluorophosphate (PyBOP (PF6−)) was purchased from Oakwood Chemical (West Columbia, USA).
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Infrared spectra were recorded on Nicolet 6700 FTIR (Thermo Scientific, USA). UV–vis and fluorescence spectra were measured on a Cary Eclipse 5G UV–Vis-NIR spectrometer and a Cary Eclipse fluorescence spectrophotometer (USA), respectively. Nuclear magnetic resonance and mass spectra were recorded on Bruker Ascend 400 Hz and Thermo Q Exactive in a high-resolution electrospray mode mass spectrometer, respectively. The SiO2 NP separation process was conducted using an Eppendorf Mini Spin centrifuge. A Jeol 1010 TEM instrument was used for the imaging of SiO2 NPs and SiO2-3 complex structure. Size distribution and surface charge were studied using a Malvern ZEN3600 Zetasizer instrument. Elemental analyses of the SiO2 NP surface were characterized by X-ray photoelectron spectroscopy (Thermo Scientific K-Alpha XPS). Thermogravimetric analysis (TGA) was conducted on Mettler Toledo micro balances. ICP-MS (Agilent with laser ablation capability) was used for the quantification of metal ion concentration in soil extracts and ICP-OES (Varian Vista-Pro) was applied for the Cu(II) concentration before and after the interaction with SiO2 NPs.
Synthesis of coumarin 3 and SiO2 nanoparticles
Coumarin 3 (Table 1) was synthesized via coumaric acid precursors as previously published (Nielsen and Houlihan 2004) (and detailed in the Supporting Information), followed by amide bond formation using PyBOP as a coupling agent (Scheme SI 1). The moderate yield was counter balanced by a simple work-up procedure by the removal of water-soluble by-products (SI Scheme 1) (Qian et al. 2019). After the preparation of SiO2 NPs via Stöber synthesis (Fors et al. 2013), the SiO2-3 nanohybrid was assembled as described in the “Experiment” section. The color of the as-received nanohybrid was yellow compared to the white SiO2 NPs due to the presence of coumarin 3.
Preparation of SiO2-3 nanohybrid
The preparation of coumarin 3 and SiO2 NPs can be found in the supporting information (SI 1.1–1.3). To assemble SiO2-3, SiO2 NP (50 mg) was mixed into an aqueous 250-μM suspension of 3 (100 mL). The pH was adjusted to 5 using 0.01 M hydrochloric acid, and the resulting mixture was incubated at 37 ℃ for 2 h while gently stirred. The SiO2-3 nanohybrid was separated from the mixture by centrifuging the solution at 13,000 rpm and decanting the liquid phase and retaining the supernatant. The resulting particles were washed with Milli-Q water (3 × 1.5 mL) and pH = 5 was adjusted using hydrochloric acid, to remove the non-adsorbed coumarins. The obtained nanohybrid solution was centrifuged at 13,000 rpm and dried in a vacuum oven at 50 °C.
Preparation of Cu(II), coumarin 3, and SiO2-3 nanohybrid solutions for optical measurements
CuCl2 dihydrate stock solution (1 mM) was prepared in Milli-Q water and used for further dilution as required. Coumarin 3 was dissolved in DMSO/HEPES buffer (1:9, v/v, 20 mM, pH = 5) at 80 µM. Then, 50 mg SiO2-3 nanohybrid was dissolved in 50 mL HEPES buffer (pH = 5) to form the stock solution.
Concentration dependence of fluorescence intensities
Fluorescence spectra of 3 (40 μM) was recorded at pH 5 at concentrations of 5–80 μM in DMSO/HEPES buffer (1:9, v/v, 20 mM, pH = 5) at λex = 435 nm with slit width of 5 nm. Meanwhile, for the fluorescence intensity of SiO2-3 nanohybrid, the stock solution was diluted into the concentrations from 5 to 50 mg/L using HEPES buffer (20 mM, pH = 5) and tested using the same instrument settings.
pH dependence of fluorescence intensities
Fluorescence intensity of 3 (40 μM) and SiO2-3 nanohybrid (7.5 mg/L) at λex = 435 nm with slit widths of 5 nm was recorded, respectively, across the pH range of 3–13, which was adjusted by 1 mmol NaOH and 1 mmol HCl solution.
Optical properties of coumarin 3, SiO2 NP, and SiO2-3 nanohybrid in the presence of various Cu(II) concentrations
Fluorescence spectra were recorded in the presence of CuCl2 for 3 (40 μM) in DMSO/HEPES buffer (1:9, v/v, pH = 5, 20 mM) and SiO2-3 nanohybrid (7.5 mg/L) in HEPES buffer (pH = 5) at λex = 435 nm with slit widths of 5 nm. The concentration of added CuCl2 dihydrate was 0–80 μM for 3 and 0–260 μM for SiO2-3 nanohybrid. The fluorescence intensity of SiO2 NPs (7.5 mg/L, pH = 5) was also recorded.
Quantitation of surface-adsorbed Cu(II) on SiO2 NPs and SiO2-3 nanohybrid by ICP-OES
A 30 mg/L stock solution of SiO2 NPs (10 mL) was mixed with 300 μM CuCl2 solution (10 mL) and then incubated at 37 ℃ for 2 h. The final concentration of SiO2 NPs was 15 mg/L and Cu(II) concentration was 9.4 mg/L. Then, the SiO2 NPs were separated from the mixture by centrifuging the solution at 13,000 rpm. The resulting solution (top layer) was sent to ICP-OES for the measurement of residual Cu(II) concentration. The same process was applied for the SiO2-3 nanohybrid.
Interference studies
Fluorescence intensity of SiO2-3 nanohybrid (7.5 mg/L) in HEPES buffer (20 mM, pH = 5) was recorded in the presence of selected metal chlorides, including Cu(II), Al(III), Ca(II), Cd(II), Co(II), Fe(III), Mn(II), Pb(II), Hg(II), Ni(II), Zn(II), Mg(II), Cu(I), and Fe(II); both in the absence and presence of CuCl2 (80 μM) at λex = 435 nm with slit width of 5 nm. A solution containing all these metal ions (200 μM each) was also prepared and fluorescence intensity of SiO2-3 nanohybrid was recorded under the same condition.
Cu(II) sensing in real soil samples
Surface soil samples were taken from roadsides in the west of Melbourne, Victoria, Australia. Each soil sample (2.5 g) was dispersed in water (25 mL) and shaken for 1 week to extract water-soluble constituents, including copper. The extracts were filtered through a nylon filter membrane with a pore size of 0.45 μm and analyzed for Cu using ICP-MS. The extracts were also measured for their Cu(II) concentration via fluorescence as described previously.
Results and discussion
Characterization of SiO2 NPs and SiO2-3 nanohybrid
Coumarin 3 is well-described in the literature, and its coordination with Cu(II) has been reported before (Qian et al. 2019; Jung et al. 2009). Based on the TEM images, both the SiO2 NPs (Stöber et al. 1968) and the SiO2-3 nanohybrid showed spherical and apparently identical morphology with a particle diameter of around 100 nm (Fig. S9). The presence of coumarin 3 in the SiO2-3 nanohybrid was confirmed by XPS (Fig. 1). SiO2 NPs both prior and after surface modification with coumarin 3 displayed characteristic peaks of C1s (285 eV), O1s (533 eV), Si2s (154 eV), and Si2p (103 eV), with the additional signal of N1s (~ 400 eV broad) appearing in the SiO2-3 nanohybrid, indicating the presence of 3. (Note: During the synthesis process of SiO2 NPs, there was only ammonia involved as N source which was fully removed by the washing and drying process.)
Fig. 1
XPS spectra of (a) SiO2 NPs and insert: N1s scan of SiO2 NPs and (b) SiO2-3 nanohybrid and insert: N1s scan of SiO2-3 nanohybrid
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Next, ATR-FTIR spectra of both SiO2 and SiO2-3 nanohybrid were recorded in solid state (Fig. 2, Fig. S10), where additional peaks originating from coumarin 3 (Fig. S7) were observed in the spectra of the SiO2-3 nanohybrid: stretching vibration of coumarin carbonyl group (1698 cm−1), pyridine C-N (1617 cm−1), amide N–H (1543 cm−1), and -C = C- bond in pyridine (1419 cm−1).
Fig. 2
ATR-FTIR spectra of SiO2 NPs and SiO2-3 nanohybrid in the 2000–1300 cm−1 range
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Comparison of the thermogravimetric analysis (TGA) curves recorded for SiO2-3 nanohybrid (black line) and SiO2 NPs (red line) at a heating rate of 5 °C/min (Fig. 3) provided approximation of the quantity of surface-adsorbed coumarins as 3 has a lower decomposition temperature than SiO2 NPs. For the SiO2-3 nanohybrid, weight loss of 18% (100 to 82%) was observed between room temperature to about 200 °C, attributed to the evaporation of physically bound water (Tham et al. 2020). Beyond, the SiO2-3 nanohybrid displayed 7.0% weight loss (82 to 75%) in the range from 200 to 450 °C which may be correlated to the combination of the SiO2 NP surface functional groups degrading (mainly -OC2H5 and –OH (Zhuravlev 2000)) amounting to 3% (based on the SiO2 NP TGA, 88 to 85%) and the amount of the surface bound coumarin 3 of 4% (7 to 3%). This allows for a coarse comparison between the loading capacity of the SiO2-3 nanohybrid and their fluorescence versus the use of the standalone coumarin 3. This means when 7.5 mg/L SiO2-3 nanohybrid is used in the experiments, the concentration of 3 will be about 0.8 μM but instead of an even distribution of coumarin 3 in the whole solution, now it would be adsorbed and concentrated on the surface of SiO2 NPs. This localized 3 concentration increase would then expectedly lead to changes in optical properties.
Fig. 3
Thermogravimetric analysis of SiO2 NPs and SiO2-3 nanohybrid
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Zeta potential of the SiO2 NPs was − 30.5 mV (7.5 mg/L in HEPES buffer, 20 mM, pH 5) due to the abundant and deprotonated surface silanol groups at testing pH of 5 when prepared using TEOS (Table 2) (Qiao et al. 2018). Upon surface adsorption of the coumarins to form SiO2-3 nanohybrid, the hydrodynamic size of the particles increased from 114.2 to 122.1 nm, and the zeta potential changed from -30.5 to -28.4 mV, a slight increase. Upon the addition of Cu(II) (at the final concentration of 40 μM) to the SiO2-3 nanohybrid solution, the zeta potential dropped to − 22.8 mV. As for comparison, when SiO2 NPs were treated with Cu(II) at the same concentration, the zeta potential displayed an even more obvious change to − 13.2 mV, suggesting significant surface adsorption of Cu(II) on the surface of SiO2 as expected due to the presence of negatively charged hydroxylate as well as -OH surface groups (Chen et al. 2021). Cu(II) adsorption by SiO2 surface as a reason behind improved sensing performance as it is another evidence of Cu(II) adsorbing onto SiO2.
Table 2
Hydrodynamic size (nm) and zeta potential (mV) of SiO2 NPs (7.5 mg/L) before and after surface interaction with coumarin 3 (7.5 mg/L) in HEPES buffer (20 mM, pH = 5), and in the absence or presence of Cu(II)(40 μM)
Samples
Size (nm)
Zeta potential (mV)
SiO2 NPs
114.2
− 30.5 ± 0.5
SiO2-3 nanohybrid
122.4
− 28.4 ± 0.2
SiO2-3 nanohybrid + Cu(II)
123.1
− 22.8 ± 0.4
SiO2 NPs + Cu(II)
114.8
− 13.2 ± 0.6
Fluorescence study of coumarin 3 in the absence and presence of Cu(II)
The pH and concentration dependence of the fluorescence properties of the coumarin 3 were studied in aqueous solution to find the optimum conditions for their application for Cu(II) sensing. Fluorescence intensity of the coumarin 3 was studied across the pH range from 3 to 11 (Fig. S11 (a)) showing that, in the range of pH 5–8, the molecule exhibited the highest intensity. However, when fluorescence intensities at concentrations of 0–80 µM dissolved coumarin 3 were recorded (Fig. S11(b)), no significant differences were observed above 40 µM.
Fluorescence titration of 3 (40 µM) with CuCl2 dihydrate (0–80 µM) in DMSO/HEPES buffer (v/v, 1/9) was conducted (Fig. 4) to identify the linear range and limit of detection. Coumarin 3 responded with gradually quenched fluorescence intensities upon addition of increasing amounts of CuCl2 at the emission maximum of 481 nm (λex = 435 nm). The fluorescence intensity response of 3 within the range of 0–80 µM of Cu(II) lacked satisfactory linear correlation (R2 = 0.96, in 0–40 µM range) contrary to the previously published work (Jung et al. 2009).
Fig. 4
(a) Fluorescence spectra of 3 (40 µM, λex = 435 nm) and (b) fluorescence intensity changes of 3 with addition of CuCl2 from 0 to 80 μM in HEPES/DMSO buffer (20 mM, pH 5)
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Fluorescence study of SiO2-3 nanohybrid in the absence and presence of Cu(II)
Fluorescence of the SiO2 NPs was negligible in HEPES buffer (20 mM, pH = 5, λex = 435 nm) (Fig. S12). To find the optimal working parameters, the fluorescence intensity of the SiO2-3 nanohybrid was recorded at the excitation wavelength of 3, 435 nm, and across the pH range of 3–11 (Fig. S13(a)). The highest fluorescence emission intensity occurred at pH 4. In addition, various concentrations of the SiO2-3 nanohybrid (50 to 0.5 mg/L) (Fig. S13(b) and (c)) in HEPES buffer were considered. The fluorescence emission intensity of the SiO2-3 nanohybrid was highest at 7.5 mg/L.
Although the greatest fluorescence intensity was observed at pH 4, at this pH, SiO2-3 nanohybrid does not respond to Cu(II) (Fig. S14). However, at pH 4.5 or 5, the fluorescence quenching of SiO2-3 nanohybrid by Cu(II) was more significant. Thus, pH 5 was selected as the working pH in the following studies. The pH dependence of the fluorescence quenching of coumarin 3 by Cu(II) was also assessed (Fig. S15), and similarly, the extent of fluorescence quenching was greater with increasing basicity of the solution in the presence of equal amounts of Cu(II) added.
Fluorescence titration of both 3 (40 μM) and SiO2-3 nanohybrid (7.5 mg/L) with CuCl2 (0–260 µM) in HEPES buffer was conducted (Fig. 5). The SiO2-3 nanohybrid displayed linear correlation between fluorescence quenching and Cu(II) concentration over the 0.033–55 and 55–260 μM ranges (R2 = 0.98), though the sensitivities (slope) (inset of Fig. 5) were different for each range. This is a valuable extension of the sensing range when compared to coumarin 3, especially as the regulatory limit for Cu(II) in drinking water is specified by the Australian Drinking Water Guidelines at 30 µM (NHMRC N 2011) and 10 mg/kg in soil. The LOD was 0.033 μM, tenfold lower than that of coumarin 3 at 0.48 μM.
Fig. 5
Fluorescence emission intensity change (λex = 435 nm) of coumarin 3 (red, 40 μM) and SiO2-3 nanohybrid (black, 7.5 mg/L, n = 3) upon the addition of various concentrations of CuCl2 in HEPES buffer (pH = 5, 20 mM)
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Interference study of SiO2-3 nanohybrid
To study the influence of other metal ions on the Cu(II)/SiO2-3 nanohybrid interaction, interference study was conducted with a variety of metal ions applied at an excess concentration (200 μM each) to SiO2-3 nanohybrid (7.5 mg/L) and Cu(II) (80 μM). Such additional ions were Al(III), Ca(II), Cd(II), Co(II), Fe(III), Mn(II), Pb(II), Hg(II), Ni(II), Zn(II), Mg(II), Cu(I), and Fe(II). Fluorescence emission spectra were examined with and without the addition of Cu(II) (Fig. 6). The measurements indicated no significant interference apart from Fe(III), which means the selectivity of 3 was successfully preserved.
Fig. 6
Fluorescence emission intensities (λex = 435 nm) of SiO2-3 nanohybrid (7.5 mg/L) in the presence (red bars) and absence (black bars) of Cu(II) (40 μM) when pre-mixed with various cations (200 μM) in HEPES buffer (pH = 5, 20 mM, n = 3)
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Cu(II) adsorption by SiO2 surface as a reason behind improved sensing performance
ICP-OES was employed to determine the amount of Cu(II) adsorbed on both SiO2 NPs and SiO2-3 nanohybrid (both presenting (-OH/-O− surface groups) using the back-titration procedure described in the “Experiment” section (Table S1). SiO2 NPs and SiO2-3 nanohybrid were used at 15 mg/L, respectively, with Cu(II) added at 9.4 mg/L. ICP-OES results showed the residual Cu(II) in the mother liquor as 7.0 mg/L in both cases. This means the SiO2 NP has a 2.4 mg/L Cu(II) adsorption. This is slightly lowered when coumarin 3 is adsorbed in SiO2-3 nanohybrid at a ~ 4% (weight ratio). Hence, interaction of Cu(II) with both the coumarin 3 and the SiO2 NP surface results in the less steep slope/increased sensitivity of the sensing observed upon addition of Cu(II) when compared to that of the coumarin 3 itself (Fig. 5). This results in an extended linear detection range of SiO2-3 nanohybrid for Cu(II) sensing in the range of 0.033–260 μM. Moreover, approximately tenfold less amount of coumarin 3 was present in the SiO2-3 nanohybrid versus when coumarin 3 used alone. This allows for less organic dye to be used while achieving improved applicable sensing range.
Application of SiO2-3 nanohybrid on real soil samples
Roadside surface soil samples were collected in the western parts of Melbourne, Victoria, Australia (Li et al. 2016). The soil samples were dispersed in water and digested while shaken for 7 days to ensure maximal dissolution of all the soluble metals. The soil samples were analyzed for their Cu content by the industry standard quantification: inductively coupled plasma mass spectrometry (ICP-MS). Next, fluorescence titration using 15 mg/mL SiO2-3 nanohybrid (1.5 mL) was conducted at pH 5. Then the soil extracts (1.5 mL) were then merged with the SiO2-3 nanohybrid solution resulting in a concentration of 7.5 mg/mL. The background fluorescence of the soil extracts is shown on Fig. S16. Fluorescence intensities were converted into Cu(II) concentrations based on the titration curves (Fig. 5). The results obtained were then compared against the ICP-MS results. The recovery of Cu(II), directly from the water extract without further sample preparation, was between 105 and 94% for all three soil samples tested (Table 3) (Qian et al. 2019).
Table 3
Determination of Cu(II) concentration and recovery values in soil extracts (n = 3 for each sample)
Cu(II) by ICP-MS (μM)
Fluorescence intensity
λem = 481 nm (a.u.)
Cu(II) by SiO2-3 (μM)
Recovery
Sample 1
1.25 ± 0.13
468.47 ± 2.36
1.31 ± 0.02
104.8%
Sample 2
1.38 ± 0.21
468.5 ± 1.72
1.30 ± 0.02
94.2%
Sample 3
1.73 ± 0.27
465.62 ± 1.45
1.69 ± 0.02
97.6%
Conclusions
SiO2-3 nanohybrid was designed, synthesized, characterized, and evaluated for their use for Cu(II) sensing in aqueous media while omitting any organic solvent carrier (required for 3 used alone). SiO2-3 nanohybrid exhibited equal selectivity towards Cu(II) in the presence of other multivalent metal ions compared to coumarin 3. The sensing signal of SiO2-3 nanohybrid versus Cu(II) concentration was linear in the range of 0.033–260 μM, which is in good correspondence with the relevant WHO and Australia standards for Cu(II) content in soil and water, and even intra-cellular imaging. This linear sensing range of the nanohybrid was significantly extended when compared to that of 3 (0–20 μM) from μM to nM level. The limit of detection in aqueous media was 0.033 μM, a more than tenfold improvement from LOD 0.48 μM of 3. SiO2-3 nanohybrid also outperforms some of the previously reported surface-modified SiO2 NP systems used for Cu(II) sensing (Table S2).
The impact of this work is the demonstrated (i) improvement of Cu(II) sensing performance; (ii) use of tenfold reduction amounts of organic molecular probe used when loaded onto the SiO2 nanoparticles while achieving superior sensing performance; and (iii) omission of organic solvents, when SiO2-3 nanohybrid was used and compared to 3 alone. This approach offers reduced cost of synthesis and potentially the amount of (in some cases toxic) organic dye used. SiO2-3 nanohybrid was validated on real soil samples against the industry standard ICP-MS technique and showed satisfactory accuracy. Therefore, the concept of SiO2-3 nanohybrid offers a convenient and customizable strategy for the use of organic sensing probes in fully aqueous media with the added benefit of improved optical sensing properties, such as linear sensing range and detection limit.
Author contribution
Bin Qian: conceptualization, methodology, formal analysis, writing—original draft, writing—review and editing. Linda Váradi: conceptualization, methodology, writing—original draft, writing—review and editing, supervision. Shamali De Silva: formal analysis. Suzie M Reichman: methodology, supervision. Lei Bao: supervision. Adrian Trinchi: writing—review and editing. Minbo Lan: writing—review and editing. Gang Wei: supervision. Ivan Cole: conceptualization, methodology, writing—review and editing, supervision, funding acquisition.
Acknowledgements
Bin Qian would like to show his appreciation to Jo Cosgriff (CSIRO) for her assistance with the NMR and MS analyses, to Paul Morrision who provided the ICP-MS results, and to Winston Liew for ICP-OES results.
Declarations
Conflict of interest
The authors declare no competing interests.
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