Preparation and Characterization of Cerium (III) Doped Captopril Nanoparticles and Study of their Photoluminescence Properties

2.1 Materials and instruments

Captopril was received as gift samples from the Iran Drug Company. Captopril drug was produced as an active substance and without impurities. Ce(NO₃)₃ with a purity of 98.5% was purchased from Merck. CAP-NP was prepared by ball milling (satellite “Pulverisette 7 classic line” (Fritsch GmbH, Germany). The surface of the product was characterized by scanning electron microscopy (SEM) model AIS2100 from Soren technology and prior to analysis the sample was covered by thin layers of gold. A Philips X-pert x-ray diffractometer with Cu Kα (λ = 1.54 Å) radiation was used to study the CAP-NPs at room temperature. Emission spectra of Ce³⁺ doped CAP-NP and CAP-NP were collected using a fluorescent spectrophotometer (Cary Eclipse 4000) at excitation wavelength of 274 nm.

2.2 Synthesis of Ce³⁺ doped CAP-NP

To prepare the nanoparticles, 5 g of captopril drug were ball-milled at 180 RPM in an iron jar for 15 hours at an optimum weight ratio of ball to powder. The CAP-NP was kept in the dark glass to avoid any decomposition reactions. Then 10% Ce(NO₃)₃ was added to the CAP-NP and they were ball-milled at 180 RPM for a further 5 hours.

2.3 Characterization of CAP-NP and Ce³⁺ doped CAP-NP

X-ray diffraction (XRD) was used to determine the CAP-NP size by using the FWHM obtained from the largest peak (Fig. 1). (Due to the low amount of cerium as the doping agent, XRD spectra showed no change after doping cerium to captopril so presence of cerium examined with photoluminescence spectra). FTIR has been used to investigate possible interactions of functional groups between CAP-NP and Ce³⁺ doped CAP-NP in the range 400 to 4000 cm^-1 (Fig. 2). Morphology of CAP-NP and Ce³⁺ doped CAP-NP were investigated using scanning electron microscopy (SEM) – Figs. 3a and 3b. PL spectra of Ce³⁺ doped CAP-NP and CAP-NP shows bands at 680 and 357 nm compared to Ce(NO₃)₃ PL spectra, as shown in Fig. 4.

Figure 1

X-ray diffraction pattern of CAP-NP

Figure 2

FT-IR spectrum of a)CAP-NP and b)Ce³⁺ doped CAP-NP.

Figure 3

(a) SEM pictures of CAP-NP with magnification 7.0 kv (b) Ce³⁺ doped CAP-NP with magnification

Figure 4

PL spectrum from (a) Ce³⁺ doped CAP-NP, (b) CAP-NP, (c) Cerium nitrate at 274 nm excitation.

3.1 Powder X-ray diffraction (XRD)

Fig. 1 shows the XRD pattern of CAP-NP prepared by the planetary high-energy ball mill process. According to the results of X-ray diffraction the size of the CAP-NP nanoparticles was calculated from the full width at half maximum (FWHM) technique using Scherer’s formula. D = K λ/(β cos θ) where K is the constant (0.99), λ is the wavelength of Cu-Kα (1.54 Å) line, β is the FWHM and θ is the diffraction angle. Size of CAP-NP obtained in the range of 34 nm.

X-ray diffraction peaks are much broader when particle size is smaller than 100 nm. CAP-NP has a crystalline structure, and exhibits additional sharper peaks around 2θ = 19.67°.

3.2 Analysis of Fourier transform infrared spectroscope(FTIR)

We reported the synthesis of Ce³⁺ doped CAP-NP and it was confirmed by FT-IR (Fig. 2b). The Ce³⁺ doped CAP-NP were studied and compared to CAP-NP in the range of 400 to 4000 cm^-1, Fig. 2a. In the CAP-NP, the peaks at 2979 and 2877 cm^-1 were assigned to the asymmetric CH₃ and CH₂ stretching vibration, and the peak at 2634 cm^-1 was due to the symmetric CH3 stretching mode. The peak at 2567 cm^-1 corresponded to the SH stretching vibration [15]. The peaks at 1747 and 1593 cm^-1 were assigned to the C=O stretching vibration of carboxylic acid and amide band, respectively. The peaks at 1471 and 1385 cm^-1 were due to the asymmetric and symmetric CH₃ bending vibrations, respectively. The peak at 1330 cm^-1 was assigned to the OH bending vibration. The peaks at 1228–1200 cm^-1 also corresponded to the C-O and/or CN stretching vibrations [16,17]. The peak at 3424−3474 cm^-1 assigned to the O-H stretching vibration. This peak was broad because of the presence OH bond of Ce(NO₃) × 6H₂O in Fig. 2b and compared with CAP-NP in Fig. 2a that indicated that there were some changes in the structure due to the presence of cerium.

3.3 Scanning electron microscopy (SEM)

SEM was used to investigate solid state of physical structure of the prepared CAP-NP. Fig. 3a is a SEM image of the CAP-NP that confirmed the surface morphology is similar to the spherical particles with a uniform distribution. Due to the high surface energy of the fine particles being created, particles have stuck together and formed agglomerates. The spherical morphology obtained by cold welding process is similar to the morphology obtained by the co-precipitation method [18]. Fig. 3b is a SEM image of the Ce³⁺ doped CAP-NP that confirmed the surface morphology is similar to the spherical particles with some of particles showing an irregular edged shape. There is a considerable agglomeration of particles. Presence of cubic structure among the accumulated spherical particles of CAP-NP has been identified.

3.4 Photoluminescence (PL) Results

Cerium is a three valent mineral material and has a 4f¹ 5d⁰ ground state and a 4f⁰ 5d¹ first excited state. The ground state of Ce³⁺ split in two sub bands of ²F_5/2 and ²F_7/2. Emission from Ce³⁺ is according to dipole transition of 4f-5d, which has a permitted parity and spin with a fairly strong vibration force [19–21].

PL spectra from Ce³⁺ doped CAP-NP were recorded at room temperature with 274 nm excitation, shown in Fig. 4a. There are two UV and visible emissions at 357 and 680 nm, which is ascribed to the well-known 5d–4f emission band of the cerium ion. The intense peak at 680 nm can be assigned to the 5D₀–7F₀ transition. The PL broad band at 357 nm is due to the cerium ion transitions ²D_3/2–²F_5/2 and ²D_3/2–²F_7/2 [22]. Also a strong peak in 547 nm that is characterized by CAP-NP. However, these features were not able to be detected from undopped captopril (Fig. 4b). Comparison of the spectra shows that these features are related to the Ce³⁺ luminescence centers present in the doped material. Due to the presence of cerium in captopril crystalline structure the cerium emission intensity increased. The PL spectra of Ce³⁺ doped CAP-NP and CAP-NP and Ce(NO₃)₃ are compared in Fig. 4.