Zn-MOF-NPs characterization
The Zn(II)-MOF-NPs was synthesized via a simple reaction of zinc acetate with nano organic linker according to the reaction scheme (Fig. S1). Off-white precipitate is obtained, filtered, then washed and finally dried. The elucidated suggested structure using the acquired micro/analytical data and were investigated as follows:
FE-SEM/EDX and HR-TEM spectroscopy
The Zn(II)-MOF-NPs FE-SEM & EDX images are shown in (Fig. 1 a, b, c). The Zn(II)-MOF-NPs morphology represented by FE-SEM images at diverse magnifications (Fig. 1 a, b) seemed to be aggregates of irregular square shapes (1 a) and almost irregular square wood shapes with average size about 118 nm (1b). Whereas, the mapping analysis using EDX (Fig. 1 c and Table 1) of Zn(II)-MOF-NPs showed the presence of (carbon/oxygen/nitrogen/zinc) as an elements construction block in each single particle. The excellent distribution of block elements alongside the cross-section displayed by EDX mapping (Fig. 1 c) confirmed the Zn(II)-MOF-NPs structure formation. Likewise, from EDX Table (Table 1) the mapping elements percentage were in a good conformity with the element’s percentage calculated theoretically: Theoretically; C, 40.18; N, 6.83; O, 28.99; Zn, 18.23; Found: C, 40.00; N, 6.57; O, 34.87; and Zn, 18.56. The TEM image of the Zn(II)-MOF-NPs appears irregular square nonosheets with average size about 120 nm, (Fig. 1 d). Whereas, (Fig. 1 e, f) shows the SAED image of 3D nanostructure of square sheets.
Elemental analysis
The Zn(II)-MOF-NPs CHN-elemental data compared with obtaining by EDX mapping and theoretically calculated were represented in (Table 1). The results were in an excellent conformity with the proposed chemical formula; the Anal. Calc.(%): C48H82N7O26Zn4, (1434.73 g/mol), C, 40.18; H, 5.76; N, 6.83; found C, 39.96; H, 5.95; N, 6.82; with a reaction yield about 87.8 %.
Mass spectrum
The Zn(II)-MOF-NPs mass spectrum (Fig. S2) and suggested fragmentation-scheme (Fig. S3) were represented. Fig. S2, showed the m/z peaks which were completely agreed with the proposed empirical-formula (C48H82N7O26Zn4) theoretically calculated and confirmed from CHN-analysis (molecular ion peak at 1434.73 m/z). The subsequent mass fragmentations as represented in Fig. S3, the ion of m/z = 1434.73 subsequent fragmentation, many main peaks at m/z= 943, 716, 488, 229, 171, 149, 108 and 65 due to losing of (ethanol and water molecules) followed by further decomposition of organic skeleton. Mostly, the sequential fragmentations of the Zn(II)-MOF-NPs were totally consistent with the mass spectrum and with the calculated theoretical fragmentations and with the assumed molecular structure.
FT-IR and UV-vis spectra
The Zn(II)-MOF-NPs IR-spectrum compared with nano organic liker is shown in (Fig. S4). The peaks at 3440, 3140, 3018, 2928 and 1157 cm-1 are due to stretching of amine groups, water and ethanol molecules of the compound [43]. The peak at 3263 cm-1 is apportioned to the stretching of N-H. The sharp-bands at 1640, 1585, 1560, 1476 and 1364 cm-1 are apportioned to stretching of C=O, C=N, and C=C [26, 44]. The bands between at 1024 and 771 cm-1 are apportioned to CH. The bands at 540 and 418 cm−1 apportioned to the coordination and covalent bonding of zinc ion with O and N [ν(Zn–O), (Zn<−N)], respectively [44–46]. The appearing of the above new bands approved the chelation between the zinc ion and nano organic linker through the N and O [44–46]. A comparison of the electronic-reflection spectra (Fig. S5) and bandgap energy (BGE) (Fig. S6) of the Zn-MOF-NPs with nano organic linker were investigated. From Fig. S5, it observed that the Zn-MOF-NPs displays various reflection bands at 235, 271, 330, 385, 408 and 634 nm, these bands may be due to LMCT and intra-ligand π- π*, n-π* [47]. Moreover, from the BGE spectra Fig. S6, it can be noted a decrease in the BGE values of the MOF to 1.79, 2.95, and 3.50 eV than linker due to the conjugation within the organic skeleton of the linker leads to rise of the BEG for HOMO valance [26, 36]
XRD and XPS analysis
The Zn-MOF-NPs XRD spectrum in comparison with Zn-MOF published-reports [48, 49] was introduced in (Fig. 2 a). The Zn-MOF-NPs XRD patterns displayed sharp-peaks proved the Zn-MOF-NPs crystalline-phase was progressed. Furthermore, the diffraction patterns matched with standard ZIF-8 and some prepared Zn-MOFs XRD patterns that demonstrated the efficacy synthesis of Zn MOF-NPs [48, 49]. The details of the XRD data estimated using Scherrer-equation were presented in (Tables S1 and S2). The results revealed that the crystallites size about 120 nm which confirmed SEM and TEM results [50].
The XPS-analysis of the synthesized Zn-MOF-NPs sample is represented in (Fig.2 b:e) proved the presence of C, O, N, and Zn without any impurities in the sample. The XPS spectrum of Zn 2p (Fig. 2b) showed a signal at 1020.92 eV ascribed to Zn(II) 2p3/2 satellite peak to prove the existence of Zn2+ in the Zn-MOF sample [51]. The O 1s spectrum (Fig. 2c) showed 3-peaks of (O-Zn-O, C-O and C=O) at (530.34, 531.25, and 533.22 eV), respectively [52–54]. The N 1s spectrum (Fig. 2d) showed a satellite peak at 398.16 eV. Finally, the C 1s spectrum (Fig. 2e) showed 3-signals at (282.85, 286.52, and 289.59 eV) are appointed to (C-C, C-N, and C=O), correspondingly [51].
Thermal analysis and thermal stability of the Zn(II)-MOF-NPs
The thermal behavior of Zn-MOF-NPs (TGA/DSC) plots (Fig. 2f) implied that the Zn-MOF-NPs underwent four breakdowns’ stages. The first and second weight loss due to the loss of C2H5OH and intra/inter H2O molecules in a temperature between 65 to 226.0 oC [55]. The third and fourth decomposition stages due to the breakdowns of organic skeleton in a temperature started from 440.0 °C. The remaining residue is zinc about (18.11 %), the discussed data was in conformity with obtaining from mass spectrometry and XRD data [56, 57]. Moreover, from the thermogravimetric behavior of the Zn(II)-MOF-NPs the results indicate the formed MOF is stable thermally at high-temperature reached to 440.0 °C [56, 57].
Based on the above discussed data, it can be deduced the 3D-structure of the Zn-MOF-NPs (Fig. 3 a), and the advanced-molecular-surface (Fig. 3 b).
Photoluminescence study & applications
The PL excitation spectrum against an emission spectrum of the Zn-MOF-NPs via auto-scan mode was presented in (Fig. 4a), the PL Zn-MOF-NPs spectrum showed an emission band at 366 nm at excitation wavelength 365 nm. Moreover, the PL spectrum of the Zn-MOF-NPs at different excitation wavelengths was recorded and presented in (Fig. S8). The fluorescent performance of the Zn-MOF-NPs may be due to intra-ligand n/π - π* transition, and molecular-orbital transitions within ligand-metal charge transferring (LMCT). On the other hand, Zn-MOF-NPs were used as optical-biosensors for detection of PSA and quantification. The Zn-MOF-NPs PL spectrum (at λex=305 nm) (10.0 mM) was investigated against a series of PSA concentrations and the results were presented in (Fig. 4b). As represented in (Fig. 4b); via increasing the PSA concentrations, the Zn-MOF-NPs PL intensity was remarkably increased in a PSA range of concentration (0.1 fg/ml to 20 pg/ml) with a slight red-shift (6.0 nm) from 366 nm to 372 nm. The results proved that the Zn-MOF-NPs could be considered as a promising biosensor based on spectrofluorimetric phenomena for detection PSA and quantification after considering the method evaluations and statistical parameters.
Method validation and analytical merits
The full calibration curve linking the PL intensity of Zn-MOF-NPs at λem=372 nm, and the concentration of PSA in a range between 0.1 fg/ml to 20 pg/ml were represented in (Fig. S9). Under ideal conditions for PL measurements, the linear-dynamic relationship for Zn-MOF-NPs PL biosensor (Fig. 4c) was obtained. From (Fig. 4c), the spectral PL intensities were strongly dependent on the rising in concentration of PSA. The calibration graph showed a well relationship (linear curve) over a range between (0.1 – 1000.0 fg/mL). The fitting enhancement equation can be stated as:
Zn-MOF-NPs PL intensity = 523.01 + 105.21 log[PSA] with r2 = 0.983.
The LOD for Zn(II)-MOF-NPs optical photoluminescence biosensor was estimated to be 0.145 fg/mL, while the LOQ was 0.438 fg/mL. The brief of the analysis of PL regression data is presented in Table S3. From Table data, the LOD / LOQ low values and wide linear concentration ranges for the proposed optical photoluminescence biosensor a validation for sensitivity. Additionally, comparing the performances of the current biosensor with other previous literature reports for the PSA quantification and determination was presented in (Table 2). From this Table, the present optical photoluminescence biosensor showed a lower LOQ / LOD, and wide linear detection PSA ranges in comparison with previous methods.
The selectivity of the present optical photoluminescence Zn(II)-MOF-NPs based biosensor was investigated to evaluate its ability to respond primarily to PSA in the existence of interfering analytes as some tumor biomarkers and different biomolecules. The PL spectrum of Zn(II)-MOF-NPs (10 mM) was measured against PSA (0.1 pg/mL), CEA “carcinoembryonic antigen” (10 ng/mL), AFP “alpha-fetoprotein” (10 ng/mL), CK-T “creatine kinase total” (10.0 ng/mL), CK-MB “creatine kinase muscle\brain” (10.0 ng/mL), glucose (100 mg/dL), uric acid (10 mg/dL), starch (10.0 M), bilirubin (10.0 mg/mL), cholesterol (10.0 mg/mL), mixture of matrix. The results of selectivity study were summarized and presented in a histogram (Fig. 4d). From the histogram it can be noted that a remarkable enhancement in the Zn(II)-MOF-NPs PL intensity with slight redshift was observed in case of PSA whereas, no observed any response for the different interfering matrix. Therefore, we can conclude that the Zn(II)-MOF-NPs is exceptionally specific and selective for PSA over other interfering matrices.
The precision, accuracy, recovery, and applicability of proposed optical photoluminescence biosensor based Zn(II)-MOF-NPs to detection of PSA and quantification in real serum samples was investigated. This study was performed via spiking of different concentrations of PSA standard (1.0, 100.0, 500.0, 1000.0 and 2000.0 fg/mL) to real serum samples and each test repeated three times, and the results of investigation were summarized in (Table 3). From the Table data and statistical evaluations of results, the mean values (X) were in target, the lower values of relative-error present (RE %) average 1.01% and relative standard deviation (SD) average 4.27 % reflect the accuracy and precision of the proposed method. Additionally, the average percent recoveries (RC %) was about 99.02% and this means that the current optical photoluminescence biosensor may be used for the detection of PSA as an important cancer biomarker diagnosis for prostate at ultra-low concentration levels with sufficient accuracy and precision.