Introduction

In recent years, the fluorescent sensors as a powerful optical analytical technique for the detection of low level of various analytes such as anions and metal cations have been progressively developed due to the simplicity, portability, cost effective and selectivity for monitoring specific analyte in environmental and biological systems [1–4]. Among various ions, Fe3+ is one of the most investigated analyte in the field of fluorescent sensor [5, 6] because it is an abundant essential ion in biological system and plays crucial rules in living organism activities, typically in the storage and transport of oxygen to tissues. However, both its deficiency and excess accumulation are associated to the various disorders such as anemia, damage to the liver and kidney and Alzheimer’s and Parkinson’s diseases [7–10]. Indeed, there are very promising Fe3+-fluorescent sensor which were prepared in a simple procedure and were able to detect this ion in trace amount [11–14]. Since Fe3+ is mostly occurred in aqueous media, it is of great importance that the sensor be able of on-line detection of this ion directly in water. To days, most of the reported Fe3+-fluorescent sensors required organic solvents for optimal performance, for example DMF [15, 16], methanol [17, 18], acetonitrile [19, 20], and THF [21], which limited their direct applicability in pure water, Thus, there is still room for the development of new class of fluorescent sensors for Fe3+ to overcome this shortage.

Very recently, covalent immobilization of fluorophore groups onto the inorganic nanomaterial substrates, as organic-inorganic hybrid optical sensor, have been introduced into the fluorescent sensor field. Among various nanomaterials, nanoporous silica materials have recently become the first nominate for preparing hybrid optical sensors because they offer large pore size, hydrothermal stability, and simple and rapid approach in monitoring analytes due to the their uniform channels with high specific surface area [22–28]. The presence of abundant OH active groups on the surface of silica nanomaterials and availability of various organosilica functionalities, as a linker, permit the attachment of a wide variety of fluorophores onto their surface. In particular, SBA-15, a two-dimensional hexagonal arrangement of cylindrical pores, is a promising one to be used as inorganic support owing to tunable and larger pore sizes facilitating diffusion of analytes through channels. In addition, higher hydrothermal stability of SBA-15 makes it a durable support in various aqueous environment with varied pH without collapsing in structure. Moreover, since both water and organic solvent can diffuse into the channel of nanoporous silica materials, the solubility properties of grafted fluorophores become solvent independent, thereby, hybrid optical sensor based on SBA-15 can be directly applied in aqueous media even if it contains water insoluble fluorophores.

In this sense and as a part of our continuing interest in the development of optical sensors for Fe3+ [29, 30], a naphthalene derivative as flourophore was covalently grafted onto the SAB-15 surface via 3-isocyanatopropyl-triethoxysilane as a linker for preparing a novel selective and sensitive hybrid optical sensor for the detection of Fe3+. In this system, the urea unit acted as a binding site for absorbing Fe3+ ions.

Materials and Instruments

Tetraethylorthosilicate (TEOS), pluronic P123, hydrochloric acid 35 %, tetrahydrofuran (THF), 3-isocyanatopropyl-triethoxysilane, 1-amino-naphthlene, Metal nitrate salts (except for FeSO4). All above materials were used without any further purification. The deionized water was used in all procedures. Low-angle X-ray scattering measurements were performed on an X’Pert Pro MPD diffractometer using Cu K α radiation (λ = 1.5418 Å). N2 adsorption-desorption isotherms were obtained using BELSORP-miniII instrument at liquid nitrogen temperature (−196 °C). All samples were degassed at 100 °C before performing measurements. The Brunauer-Emmet-Teller (BET) and Barrett-Joyner-Halenda (BJH) equations were applied on sorption data using BELSORP analysis software to calculate physical properties of materials such as specific surface area, pore diameter, pore volume and pore size distribution. The Fourier transform infrared (FT-IR) spectra of samples were recorded on a RAYLEIGH WQF-510A apparatus. Transmission electron microscopy (TEM) was performed on Zeiss EM900 instrument at an accelerating voltage of 80 kV. Samples were dispersed in ethanol using an ultrasonic bath and a drop of the ethanol mixture was placed on a lacey carbon-coated copper grid for analysis. Thermogravimetric analysis (TGA) was carried out in a TGA Q50 V6.3 Build 189 instrument from ambient temperature to 1000 °C with a ramp rate of 20 °C/min in air. Fluorescence spectra were recorded on Agilent G980A instrument.

Preparation of SBA-15

SBA-15 was prepared based on Ref [31]. A homogenous solution of 11.7 g Pluronic P123, 303.4 g water, 73.3 g HCl 37 % was prepared followed by stirring for 3 h at 55 °C. Next, 25 g TEOS was added under stirring and solution was remained at static condition for 24 h at 55 °C. Afterwards, the reaction batch was kept in oven for another 24 h at 100 °C. The product was washed with HCl/ethanol mixture and finally calcined under air for 3 h at 550 °C.

Preparation of SBA-NCO

In a typical procedure, 5 mmol 3-isocyanatopropyl-triethoxysilane and 5 mmol 1-amino-naphthalene was dissolved in 50 ml THF and refluxed for 6 h. Then, 1 g SBA-15 was added to the container and mixture was refluxed for another 24 h. Next, the solid was filtered and washed with an excess amount of THF and ethanol. Finally, the pale pink solid was dried overnight. Scheme 1 depicts the synthetic procedure of SBA-NCO.

Scheme 1
scheme 1

Synthetic procedure of SBA-NCO

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Results and Discussion

Characterizations

The powder XRD patterns of original and functionalized SBA-15 are shown in Fig. 1. All of the materials exhibited the characteristic high intensity peak related to the diffraction from 100 plane near 2θ value of 1°. In addition, they showed two other significant peaks related to the diffraction from 110 to 200 planes near 2θ value of 2° corresponding to the well-ordered one-dimensional hexagonal mesoporous channels of SBA-15. Observing these peaks in SBA-NCO sample indicated that original structure of SBA-15 remained intact after functionalization steps. Nevertheless, lowering the intensity of those diffraction were attributed to decrease in long-range order resulted from grafting of the organic moieties onto the surface of SBA-15. The inset of Fig. 1 shows the TEM image of SBA-15 and the ordered arrays of channels of SBA-15 with pore diameter about 6 nm can be clearly seen which supported XRD result.

Fig. 1
figure 1

Low angle powder XRD patterns of (a) SBA-15 and (b) SBA-NCO (inset: TEM image of SBA-15)

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The FT-IR spectra of SBA-15 and SBA-NCO are shown in Fig. 2. The wide band around 3360 cm−1 in both samples was related to the strong vibration of -OH groups on surface. A series bands within the range of 2880–2960 cm−1 were due to the stretching vibrations of methylene -(-CH2-)- in propyl chain. The bands around 1650 and 1565 cm−1 were attributed to the vibrations of physiosorbed water molecules plus C = O stretching vibrations and the bending vibrations of -NH- groups indicating the formation of amide groups. The weak bands around 1400 and 1445 were assigned to the C-N and ring C = C stretching vibrations, respectively. The other remained bands around 800, 960, and 1100 cm−1 were attributed to the Si-O-Si symmetric stretching, Si-OH symmetric stretching, and Si-O-Si asymmetric stretching vibrations, respectively. Therefore, the FT-IR results supported the successfully covalent grafting of organic moieties onto the SBA-15 surface.

Fig. 2
figure 2

FT-IR spectra of (a) SBA-15 and (b) SBA-15-NCO

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Textural properties of SBA-15 and SBA-NCO were further investigated using N2 adsorption-desorption experiment and corresponding isotherms are presented in Fig. 3. Both samples showed type IV isotherms with H1-type hysteresis loops which was characteristic of well-developed uniform honeycomb mesoporous materials according to the IUPAC classification [31] confirming XRD results about maintaining mesoporous structure after functionalization step. Decrease in the volume of adsorbed N2 gas in SBA-NCO was due to the grafted organic moieties onto the SBA-15 surface which consequently led to the reduction of pore sizes, supported by the provided data in Table 1. As expected, the quantity of all three properties in Table 1 were decreased in functionalized SBA-15.

Fig. 3
figure 3

N2 adsorption-desorption isotherms of (a) SBA-15 and (B) SBA-NCO

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Table 1 Textural properties of SBA-15 and SBA-15-NCO
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Fluorescence Response of SBA-NCO in Water

In SBA-NCO, urea and naphthalene units acted as a binding site for analytes and fluorophor groups, respectively. In all fluorescence experiments, SBA-NCO was well suspended in water by sonicator to form 0.2 g.L−1 monodispersed solution. This suspension was initially tested toward a wide range of metal cations (Mn+) including Na+, Mg2+, Al3+, K+, Ca2+, Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Pb2+. For this, the fluorescence response of the suspended SBA-NCO (3 ml) in the presence of Mn+ (3 × 10−4 M) was recorded following excitation at 295 nm with excitation and emission slits of 5 nm. As shown in Fig. 4, SBA-NCO exhibited a strong fluorescence emission with maximum located at 380 nm emitted from naphthalene groups. In the presence of Fe2+, insignificant change in fluorescence intensity was observed, while minimal or no changes were observed for other investigated cations except for Fe3+. As observed in Fig. 4, the fluorescence intensity of SBA-NCO was drastically quenched upon addition of Fe3+. Hence, SBA-NCO exhibited high selectivity for Fe3+ over other cations. The quenching effect of Fe3+ can be related to the paramagnetic properties of Fe3+ ion when it bound to the urea unit (Scheme 2) [32, 33].

Fig. 4
figure 4

Fluorescence spectra of the suspended SBA-15-NCO (0.2 g.L-1) in the presence of cations (3×10-2 M) including Na+, Mg2+, Al3+, K+, Ca2+, Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Pb2+

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Scheme 2
scheme 2

Binding mode of Fe3+

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For further evaluation of SBA-NCO as a highly selective optical sensor for Fe3+, the competitive experiments were performed in the coexistence of Fe3+ and other cations as an interfering ions. The fluorescence spectra were recorded by addition of the suspended SBA-NCO into the mixture of Fe3+ (0.5 × 10−4 M) + Mn+ (2.5 × 10−4 M) following excitation at 295 nm. The derived results in Fig. 5 demonstrated that background metal ions, even with 5 times greater in amount, induced small or no interference with the detection of Fe3+ confirming that SBA-NCO is a highly selective optical sensor for Fe3+.

Fig. 5
figure 5

Normalized fluorescence intensity of the suspended SBA-15-NCO (0.2 g.L-1) in the presence of interfering cations (2.5×10-4 M) with Fe3+ (0.5×10-4 M)

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Figure 6 displays the changes in the fluorescence emission of the suspended SBA-NCO (0.2 g.L−1) upon addition of an increasing amount of Fe3+ following excitation at 290 nm. The amount of Fe3+ was varied within the range of 10−4–10−6 M. As shown in Fig. 6, upon treatment with Fe3+, a gradual fluorescence quenching was observed, however, the fluorescence intensity was not completely quenched which can be due to the inaccessibility of some fluorophore groups inside the channels. Moreover, the inset of Fig. 6 shows the plot of I0/I against concentration of Fe3+ confirmed good linearity between fluorescence intensity of the suspended SBA-NCO and low-level concentration of Fe3+ with r 2 = 0.9529. As seen in the inset, the deviation from linearity was observed in higher concentrations of Fe3+ which could be due to the difficulty of Fe3+ ions to reach inner fluorophores. The detection limit (DL) was calculated based on DL = 3Sd/m equation, where Sd and m represent standard deviation of the blank solution measured by 6 times and m is the slope of fluorescence intensity versus concentration of Fe3+. DL was calculated to be 4.5 × 10−6 M.

Fig. 6
figure 6

The fluorescence response of the suspended SBA-15-NCO (0.2 g.L-1) with different concentration of Fe3+. The inset is the Stern-Volmer plot

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Practical application of SBA-NCO as a successful optical sensor of Fe3+ requires the sensor to be tested in a wide pH range. The fluorescence experiments of the suspended SBA-NCO (0.2 g.L−1) were performed in the absence and presence of Fe3+ (3 × 10−4 M) in a pH values ranging from 2 to 12. The pH of the solution was adjusted using either NaOH or HCl solutions. As seen in Fig. 7, the fluorescence intensity of the suspended SBA-NCO were stable and negligibly changed in both mode in the applied pH range. Therefore, the detection of Fe3+ using SAB-NCO directly in water was pH independent further confirming SBA-NCO as a successful optical sensor for Fe3+ in pure water further.

Fig. 7
figure 7

Effect of the pH on the fluorescence intensity of the suspended SBA-15-NCO (0.2 g.L-1) in the absence and presence of Fe3+ (3×10-4 M)

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Conclusion

In conclusion, a novel organic-inorganic hybrid optical sensor (SBA-NCO) was designed and prepared though functionalization of SBA-15 using isocyanatopropyl-triethoxysilane and 1-amino-naphthalene by post grafting method. The characterization results confirmed the organic moieties were successfully introduced onto the surface of SBA-15 without collapsing in original structure of SBA-15. The fluorescence results displayed that SBA-NCO was a highly selective optical sensor for the detection of Fe3+ in pure water without requiring organic solvent even in the presence of higher concentrations of other metal cations as interfering ions and over a wide range of pH values from 2 to 12. Moreover, a good linearity between normalized fluorescence intensity and concentration of Fe3+ (r 2 = 0.9529) with the detection limit of 4.5 × 10−6 M were established.