Sudan Blue II Dye loaded modified MCM-48: Colorimetric, Paper Strip, and Fluorescent Sensor for Cu 2+ ion

doi:10.21203/rs.3.rs-3421933/v1

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Sudan Blue II Dye loaded modified MCM-48: Colorimetric, Paper Strip, and Fluorescent Sensor for Cu 2+ ion

https://doi.org/10.21203/rs.3.rs-3421933/v1

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In this work, Sudan Blue II dye is successfully grafted on the walls of chloro-functionalised MCM-48. The synthesized probe selectively detect Cu²⁺ ion by fluorescence spectroscopy. The probe designated as SB@modMCM-48 was characterized by various spectroscopic methods including FT-IR, SEM, EDX and N₂ adsorption-desorption isotherm, etc. The fluorescent intensity of the probe is quenched by 14 times in presence of Cu²⁺ ion. The fluorescent detection is reversible, selective with low detection limits. The LOD and K_a (association constant) was calculated to be 1.45 × 10^− 10 M and 5.38 × 10⁷ M^− 1 respectively with a linear range of detection. The addition of Cu²⁺ ion to the probe solution also imparted a visual color change. Paper strip sensor was also developed for naked eye detection of Cu²⁺. The low LOD limits, binding stoichiometric constant, etc. point towards high affinity of Cu²⁺ towards the nanosensor. Interestingly, the Cu²⁺:sensor complex selectively sense amino acid cysteine in its aqueous solution, with a limit of detection of 10.46 × 10^− 8 M.

Sudan Blue II

MCM-48

paper sensor

fluorescent sensor

limit of detection

In the last few decades, noteworthy efforts have been made to devise novel chemosensors for detection of biological and environmental important cations and anions [1, 2]. Copper is one such important transition metal which is also the third most important element after iron and zinc that helps in the functioning of various life processes [3]. Cu²⁺ acts as a co-factor in diverse variety of enzymes such as galactose oxidase, cytochrome c-oxidase, lysyl oxidase, Cu/Zn superoxide dismutase, tyrosinase, etc. [4, 5]. However, if present in excess or deficient amount, it causes pathological conditions in humans. Excessive amount of it causes neurodegenerative diseases such as Wilson’s disease, Alzheimer’s disease, prion disease, Menkes disease, gastrointestinal disorders, kidney damage, gastrointestinal disorders [6, 7]. Copper deficiency in the body can lead to degeneration of neurons, cardiac hypertrophy, growth retardation [8, 9]. The maximum level of copper in drinking water approved by the U.S. Environmental Protection Agency (EPA) is 1.3 ppm [10]. The foremost techniques being employed to detect Cu²⁺ are inductively coupled plasma-atomic emission spectrometry, liquid-liquid extraction flame atomic absorption spectroscopy, atomic absorption spectrometry, and inductively coupled plasma-mass spectrometry [11–13]. However, expensive instrumentations, complicated pre-treatment requirement and need for highly-trained operators make these unsuitable for routine application and monitoring. In the present day, colorimetric/fluorescent probes have gained a lot of attention due to its low cost kits, simplicity, ease of detection and better selectivity and sensitivity. In the past decades, colorimetric and paper strip sensors have garnered immense popularity due to its cost effectiveness, rapid visual eye monitoring and simple preparation methods without the help of sophisticated instruments [14].

Organic dyes play a key role in our daily life as well as in industrial application [15]. Dyes are being used as chemical and optical sensors due to their strong chromogenic ability (absorption, emission, transmission, etc.) and responsiveness towards the environment. However, dyes undergo fluorescent self quenching at higher concentration due to intermolecular collisions and aggregation of dye molecules in solution which ultimately leads to decrease in optical activity [16, 17]. To reduce these problems, dyes have been incorporated into solid matrix such as minerals, clays, polymers, mesoporous materials, etc. so that the desirable optical properties might be stabilized due to the shielding response provided by the desirable host [18]. Among the solid matrix, mesoporous materials (MCM-48) due to their porous nature, three-dimensional structure, and narrow pore size distribution and ordered structure array is considered a good host matrix for anchoring organic dye molecules. Mesoporous materials have surface silanol groups and abundant pore channels through which the dye molecules can be easily dispersed and fixed at various locations of the pore channels, thereby restricting the rotation and mobility of the dye molecule to a specific area and gradually reducing its aggregation [19, 20]. Immobilization of the dye molecules into the internal pore channels of mesoporous material leads to enhancement in sensitivity, selectivity, photostability of the dye and improves signal to noise ratio and LOD response of the dye molecules upon binding to an analyte [21, 22].

Tao and his co-workers reported a dual emission amphiphile/dye modified mesoporous silica as a fluorescent sensor for Al³⁺, Fe³⁺ and Cr³⁺ [23]. In a recent study, Han et al. reported a highly stable fluorescent probe based on mesoporous silica for selective and sensitive detection of Cd²⁺ [24]. In another report, Dong and his co-workers reported a rhodamine dye modified mesoporous material as fluorescent sensor for detection of Hg²⁺ [25]. A modified mesoporous material with an azo dye (4-[(4-isothiocyanatophenyl) azo]-N-N′-dimethylaniline) has been reported as a chromogenic sensor for Hg²⁺ [26]. A detailed study has been reported by Liu and his co-workers based on immobilization of 4-(2-pyridylazo)-resorcinol on functionalized mesoporous material for heavy metal detection [27]. Lu et al. developed fluorescent sensors based on mesoporous silica nanosphere for Cu²⁺ detection [28].

In this chapter, Sudan Blue II (SB) dye has been successfully grafted into the walls of chloro-functionalised MCM-48. The chemosensor provides a visual color change on addition of Cu²⁺ in its solution. Additionally, paper strips coated with the sensor provides rapid screening and selective detection of Cu²⁺ ion. Fluorescence spectroscopic measurements were also carried out for Cu²⁺ with high selectivity, sensitivity and low LOD range of detection. Interestingly, the Cu²⁺:sensor complex acted as a selective fluorescent probe for the amino acid cysteine in aqueous solution with low limit of detection.

Materials

MCM-48, Sudan Blue II dye, 3-chloropropyltriethoxysilane (CPTS), toluene, and all other chemicals were purchased either from Sigma Aldrich or LOBA. The chemicals were of analytical grade and used without further purification. The chemical solutions (10^− 4 M) were prepared in de-ionised water obtained from quartz double distilled plant. The FT-IR spectra were recorded in a Perkin Elmer RXI spectrometer as KBr pellets. The fluorescence and UV-Visible spectra were recorded in HITACHI 2500 and Shimadzu UV 1800 spectrophotometer respectively using quartz cuvette (1cm path length). Scanning Electron Microscopy was done using Zeiss FESEM Sigma 300 and Energy Dispersive X-ray Spectroscopy was studied using Ametek EDAX. Powder X-ray diffraction (PXRD) patterns of the synthesized samples were recorded using a Rigaku Ultima IV X-Ray diffractometer (Cu K_α radiation, λ = 1.5418 Å) at 40 kV and 40 mA. Thermogravimetric analysis (TGA) was done using Metter Toledo TGA/DSC1 STAR^e system in presence of N₂ atmosphere in the temperature range 50–700°C at a heating rate of 10°C/min. The N₂ adsorption-desorption isotherm of the synthesized samples were recorded at 77 K by using Micromeritics Tristar 3000 analyser. The samples were degassed at 110°C for 4 h prior to the measurement.

Synthesis of modified MCM-48

Chloro-functionalised MCM-48 was prepared as follows [29]: 2.5 g of MCM-48 was added to 100 mL toluene in a 250 mL round bottom flask. The mixture was then subjected to vigorous stirring for 10 minutes, followed by addition of 20 mmol of 3-chloropropyltriethoxysilane (CPTS). The solution was refluxed for 7 hours and then filtered, washed with toluene and dried overnight.

Synthesis of the sensor (SB@modMCM-48)

The sensor was prepared as follows: 1 g of the modified MCM-48 and 0.1 g of Sudan Blue II dye (SB) was taken in ethanolic solution and kept on stirring for 24 hours [30]. The solid obtained was centrifuged, washed thoroughly with ethanol and the obtained solid was oven dried at 120 ℃ for 8 hours. The sensor is designated as SB@modMCM-48 (scheme 1).

Characterization of SB@modMCM-48

The characterisations of the sensor were done by the approaches of FTIR, PXRD, N₂ adsorption-desoprtion studies, and SEM-EDX analysis. The FTIR spectra of MCM-48, chloro-functionalised MCM-48 and the dye grafted MCM-48 is depicted in Fig. 1. As well displayed, broad band around 3500 − 3000 cm^− 1 and weak bands at around 1700 cm^− 1 arises due to surface hydroxyl groups. The bands centered around 1200 and 1085 cm^− 1 are assignable to Si-O-Si stretching of the framework while the band centered around 970 cm^− 1 is assigned to Si-OH vibrations. After chloro-functionalisation of MCM-48, two weak bands appear at 2960 and 2850 cm^− 1 due to stretching of CH₂ of the propyl chain of the silylating agent. After grafting of the dye into the functionalized MCM-48, bands appear at 1350 and 1760 cm^− 1 corresponding to C-N and C = O stretching vibrations.

The low angle XRD data of parent MCM-48 exhibited a peak at 2.5^o and around 3^o corresponding to (211) and (220) plane (Fig. 2). Two low intensity peaks appear between 4^o and 5^o corresponding to (420) and (332) plane. The pattern clearly shows the presence of an Ia3d cubic mesoporous framework with well crystallinity and high long-range order. The chloro-functionalised MCM-48 also maintained the same diffraction peak but with lower intensity. The SB@modMCM-48 showed slight shift in peak position to higher 2Ө value with further decrease in peak intensity. This observation suggests the successful grafting of the dye on the walls of the mesoporous material and decrease in unit cell of the modified MCM-48 [32].

The N₂ adsorption-desorption isotherm of MCM-48 showed typical type IV monolayer/multilayer isotherm with H3 hysteresis loop which are characteristics of highly ordered mesoporous materials (Fig. 3). A sharp inflection at relative pressure p/p_o = 0.2–0.3 was observed, which indicates the capillary condensation of N₂ gas within the uniform pores of mesoporous MCM-48 material. The uniformity of pore size distribution was indicated by the height and sharpness of the inflection point. After chloro-functionalization and the anchoring of SB dye, the adsorption branches were significantly shifted toward lower relative pressure (p/p_o). The decrease in surface area, pore volume and pore diameter (Table 1) confirm the presence of SB dye on the surface and pores of the mesoporous materials which partially leads to blockage of N₂ molecules [33].

Table 1

Textural properties of MCM-48, Chloro-functionalised MCM-48 and SB@modMCM-48
Materials	Surface Area, A_BET (m²g^− 1)	Pore Volume V_BJH (cm³g^− 1)	Pore Diameter D_BJH (nm)
MCM-48	1205.31	0.48	2.52
Chloro-MCM-48	898.50	0.31	2.36
SB@moMCM-48	431.20	0.20	2.19

The SEM image of MCM-48 (Fig. 4a) shows the presence of spherical morphology which is characteristic of MCM-48 grains. However, after chloro-functionalisation, only a few changes were observed (Fig. 4b). After the grafting of the dye molecule, the morphology showed some distortion and irregularity in elliptical and spherical shapes with agglomeration (Fig. 4c). This suggests that the dye is deposited onto the surface and into the mesopores of MCM-48 moiety. The energy disperse X- ray analysis (EDAX) confirms the coating of the dye in the pores of MCM-48 sample (Fig. 4d). The energy disperses X- ray (EDAX) elemental mapping analysis also indicates the uniform distribution of the dye into the MCM-48 material. The EDAX elemental mapping of O, Si, C and N in RANH@MCM-48 is shown in Fig. 5.

The TGA curve of chloro-MCM-48 and dye grafted MCM-48 is shown in Fig. 6. As evident from the plot, the modified MCM-48 shows initial weight loss at 100 ℃ which is attributed to humidity, after which it remains constant which depicts its thermal stability. After loading of the dye, it shows two weight losses at around 140–200 ℃ and 480–550 ℃ owing to cleavage of side chain of amine group and loss of aromatic moiety respectively.

SB@modMCM-48 as colorimetric and paper strip sensor for Cu²⁺

Colorimetric sensing of Cu²⁺ by the sensor was studied under normal visible light. 20 mL of the prepared sensor solution is divided into different test tubes. To the test tubes, 10µL of different metal solutions (10^− 4 M) were added. When different concentration of Cu²⁺ ion solution were added to the sensor solution, the color of the probe instantaneously changed from blue to dirty blackish color (Fig. 7). The visible color change was observed only in case of Cu²⁺ and no other interfering metal ion solution could induce any characteristic color change to the probe (Fig. 8). Paper strip sensor was also developed for the naked eye detection of Cu²⁺ (Fig. 9). For developing paper strips, Whatman filter paper 42 was cut into small strips. These strips were wetted with 100µL of the probe solution and allowed to dry. 20 µL of different metal solutions were added to these strips and the process was repeated three times. As evident from Fig. 9, only Cu²⁺ imparted a dirty black color to the paper strip.

SB@modMCM-48 as a fluorescent sensor for Cu²⁺

5 mg of SB@modMCM-48 was taken and dispersed in 20 mL CH₃CN:HEPES buffer (1:1, v/v, pH = 6.8) followed by sonication for 10 minutes to get a fine dispersion. The UV-Vis spectrum was recorded and the excitation wavelength for the sensor was fixed at 280 nm. The solution transferred to a cuvette was excited with 280 nm photons and fluorescence emission was kept within 280–650 nm with 10 nm slit width for both excitation and emission. To this solution 50µL of particular metal salt solution (10^− 4 M) was added which makes the concentration within the cuvette 2.44×10^− 7 M. The solution was shaken well and kept to settle for a couple of minutes and then the spectra were recorded. From the bar diagram in Fig. 10, it can be seen that only Cu²⁺ quenches the fluorescent intensity of the probe to a considerable extent.

After the recognition study, fluorescent titration was carried out by gradually adding different concentration of Cu²⁺ ion to the solution of SB@modMCM-48 inside the cuvette. The initial concentration of Cu²⁺ inside the cuvette is 0.49 × 10^− 7 M on adding 10 µL of the metal solution which ultimately becomes 5.6 × 10^− 7 M on addition of 120 µL of the metal solution. The spectra showed in Fig. 11a shows a gradual decrease in fluorescent intensity of the chemosensor upon gradual addition of Cu²⁺ ion with slight red shift from 391.2 nm to 414.5 nm. The quenching of fluorescent intensity for Cu²⁺ is ca. 14 fold. To further study the spectra, the fluorescent peak intensity was plotted as a function of analyte concentration (Fig. 11b). The plot fits linearly with coefficient of determination (R²) equal to 0.996.

The stoichiometry of binding for the interaction of SB@modMCM-48 with Cu²⁺ ion was calculated from the plot of log [(I₀-I)/ (I-I_∞)] as a function of [Cu²⁺] (shown in Fig. 12a), where I₀ is the fluorescence intensity of SB@modMCM-48 in absence of Cu²⁺, I is the fluorescence intensity of SB@modMCM-48 at different added concentrations of Cu²⁺ and I_∞ is the lowest intensity of SB@modMCM-48 upon titration with Cu²⁺ ion. The slope of the plot and R² was found to be 2.133 and 0.993 which indicates 2:1 interaction between sensor and the analyte. The limit of detection (LOD) is calculated using the formula 3σ/K [34], where σ = standard deviation, K = Slope obtained from the plot of fluorescence intensity vs. concentration of Cu²⁺ (Fig. 11b). The association or binding constant (K_a) of SB@modMCM-48 with Cu²⁺ was calculated using the modified Benesi-Hildebrand equation (Fig. 12b). The LOD and K_a was calculated to be 1.45 × 10^− 10 M and 5.38 × 10⁷ M^− 1 respectively. The role of fluorescence quenching can be studied with the help of Stern-Volmer equation: I_o/I = 1 + K_SV [Q] [35], where I_o and I represent fluorescence intensity in absence and presence of the quencher and K_SV is the S-V quenching constant. The S-V plot (Fig. 12c) is found to be linear which suggests a dynamic quenching with K_sv value of 7.02 × 10⁷ M^− 1.

The UV-visible spectrum of SB@modMCM-48 in presence of Cu²⁺ was recorded. On gradual addition of Cu²⁺ to the solution of the sensor, the intensity of peak at 412 nm gradually increases while the peak at 590 nm gradually decreases (Fig. 13). This results in isosbestic point at 513 nm. On further addition of the analyte, a red shift occurs from 412 nm to 456 nm. On plotting the absorbance value with respect to analyte concentration (Fig. 14a), a linear change is obtained with R² = 0.997. Figure 14b is plotted to calculate the binding stoichiometric ratio between the sensor and the analyte. The slope of the graph log [(A₀-A)/(A-A_max)] vs. log [Cu²⁺], where A₀ is the absorbance of SB@modMCM-48 in absence Cu²⁺, A is the absorbance of SB@modMCM-48 at different added concentration of Cu²⁺ and A_max is the highest absorbance value of SB@modMCM-48 on adding one equivalent of Cu²⁺ is found to be 2.335 which confirms 2:1 binding ratio. The obtained results are in sync with those obtained from fluorescence spectroscopic measurements.

The selectivity and sensitivity of the chemosensor towards Cu²⁺ has been studied. For this purpose, fluorescent titrations were carried out adding 50 µL of different metal solutions like Na⁺, K⁺, Li⁺, Zn²⁺, Co²⁺, Ni²⁺, Mn²⁺, Mg²⁺, Fe³⁺, Fe²⁺, Pb²⁺, Hg²⁺, Al³⁺, into the solution of SB@modMCM-48 (CH₃CN/HEPES buffer, 1:1, pH = 6.8) in absence and presence of Cu²⁺ ion. As evident from the Fig. 15 in the form of bar diagram that other metal ions did not interfere in the interaction of the probe with Cu²⁺ ion.

Regeneration and reversibility are the major criteria for solid state chemosensors in practical applications [36]. For this purpose, the metal ion chelator, Na₂EDTA (10^− 3 M) was gradually added to the suspension containing 1:1 SB@modMCM-48:Cu²⁺. The fluorescence intensity (Fig. 16) was found to increase gradually upon addition of EDTA²⁻, which is due to the stability constant of Cu²⁺-EDTA is larger than that of Cu²⁺-SB@modMCM-48.

Proposed Mechanism of Binding

Upon addition of Cu²⁺ ion to the sensor solution, it binds with heteroatoms present in the chemosensor (Fig. 17). The visual color change observed as soon as the analyte is added to the solution containing the sensor can be attributed to deprotonation of the NH group upon complexation with Cu²⁺ ion. The well defined isosbestic point in the UV spectrum (Fig. 13) and the red shift suggests the conversion into a new chemical entity in solution and extension of conjugated system with complexation [37]. Due to paramagnetic nature and heavy atom effect of Cu²⁺ [38], static quenching is observed. The deprotonation of the NH group and anthraquinone to Cu²⁺ π-cation interaction causes ligand to metal charge transfer (LMCT), which could induce a non-radiative pathway deactivation of the excited state fluorophore [39]. As a result of these occurrences, spectral change as well as fluorescent quenching is observed for the sensor upon Cu²⁺ addition.

Cu²⁺:SB@modMCM-48 complex for cysteine detection

The selectivity of Cu²⁺:probe for the amino acid cysteine was studied using the Cu-S affinity. The fluorescent spectral changes were recorded by adding various amino acids and biomolecules (50µL, 10^− 3M, H₂O) to an equivalent solution of Cu²⁺:SB@modMCM-48 in CH₃CN:HEPES. Interestingly, fluorescent property of the complex changed after addition of cysteine solution. It is worthwhile to mention that other S containing amino acid like methionine did not hamper the selective detection of cysteine by the complex probe. After addition of cys, a new band at 362 nm gradually increases (Fig. 18a). This recovery in fluorescent intensity peak may be due to higher affinity of chelation of cysteine with Cu²⁺ thereby leaving the chemosensor SB@modMCM-48 free (Fig. 20). The plot of FL intensity as a function of cysteine concentration is linear in the range 0.49 × 10^− 5 M to 9.09 × 10^− 5 M. The LOD was calculated from the linear relation obtained from the plot of fluorescence intensity vs. cysteine concentration [22] (Fig. 18b). The LOD of Cu²⁺:SB@modMCM-48 for cysteine was calculated to be 10.46 × 10^− 8 M. Moreover, the binding stoichiometric ratio and binding constant were also calculated to be 1:1 and 4.82 × 10⁸ M^− 1 (Fig. 19).

The dye Sudan Blue II has been successfully grafted into the walls of modified MCM-48 and the probe was characterized by various instrumentation techniques. The chemosensor could selectively identify Cu²⁺ ion by colorimetry and paper strip sensor. The selective and reversible detection of Cu²⁺ was also carried out by fluorescence spectroscopic measurements in CH₃CN:HEPES (1:1, v/v, pH = 6.8) in presence of various metal ions. The probe could selectively sense Cu²⁺ ion by fluorescent turn-off pathway with a linear range of detection from 0.49 × 10^− 7 M to 5.6 × 10^− 7 M respectively. The subsequent limit of detection (LOD), binding constant and quenching constant were calculated to be 1.45 × 10^− 10 M, 5.38 × 10⁷ M^− 1 and 7.02 × 10⁷ M^− 1 respectively. Interestingly, the Cu²⁺:sensor complex acted as a probe for selective detection of amino acid cysteine with low detection limit.

Acknowledgements

The authors thank SAIF, Gauhati University for providing the instrumental facilities, and Dr. Sangeeta Agarwal, Cotton University, Guwahati, India for conducting the FTIR analysis.

Author Contributions

SK has done all the experiments and written the paper; DKD has done the data analysis of the results. All authors reviewed the manuscript.

Funding Statement

No funds or grants were received.

Data Availability

The results/data/figures in this manuscript have not been published elsewhere, nor are they under consideration by another publisher. The data that support outcomes of this study are available on request from the corresponding author.

Ethics Approval The authors declare that the research process did not involve any human or animal experiments.

Competing Interests The authors declare no competing interests.

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Scheme 1 is available in the Supplementary Files section.

No competing interests reported.

Onlinefloatimage1.png
Scheme 1 Grafting of SB dye onto modified MCM-48

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Sudan Blue II Dye loaded modified MCM-48: Colorimetric, Paper Strip, and Fluorescent Sensor for Cu 2+ ion

Status:

Version 1

Abstract

Figures

Introduction

Experimental

Materials

Synthesis of modified MCM-48

Synthesis of the sensor (SB@modMCM-48)

Results and Discussions

Characterization of SB@modMCM-48

SB@modMCM-48 as colorimetric and paper strip sensor for Cu²⁺

SB@modMCM-48 as a fluorescent sensor for Cu²⁺

Proposed Mechanism of Binding

Cu²⁺:SB@modMCM-48 complex for cysteine detection

Conclusion

Declarations

References

Scheme

Additional Declarations

Supplementary Files

Status:

Version 1

Sudan Blue II Dye loaded modified MCM-48: Colorimetric, Paper Strip, and Fluorescent Sensor for Cu 2+ ion

Status:

Version 1

Abstract

Figures

Introduction

Experimental

Materials

Synthesis of modified MCM-48

Synthesis of the sensor (SB@modMCM-48)

Results and Discussions

Characterization of SB@modMCM-48

SB@modMCM-48 as colorimetric and paper strip sensor for Cu2+

SB@modMCM-48 as a fluorescent sensor for Cu2+

Proposed Mechanism of Binding

Cu2+:SB@modMCM-48 complex for cysteine detection

Conclusion

Declarations

References

Scheme

Additional Declarations

Supplementary Files

Status:

Version 1

SB@modMCM-48 as colorimetric and paper strip sensor for Cu²⁺

SB@modMCM-48 as a fluorescent sensor for Cu²⁺

Cu²⁺:SB@modMCM-48 complex for cysteine detection