Molecular and fluorescence spectroscopic studies of polyacrylic acid blended with rhodamine B mixed gold nanoparticles

ABSTRACT This work studies the improvement of the emission spectra of polyacrylic acid (PAA) by adding different ratios of rhodamine dye (RhB) to different sizes and shapes of gold nanoparticles (AuNPs). AuNPs were prepared by chemical reduction method at heating temperature 80°C and a different time. The experimental results of both UV–Visible spectroscopy and TEM technique showed that the AuNPs were formed at different times (1–15 min) with different plasmonic band positions and has particle sizes 11 and 13 nm, for AuNPs were formed at 1 and 10 min respectively. FTIR, UV–Vis, X-ray diffraction, and SEM techniques used in the characterization of PAA blended by an equivalent amount of RhB and AuNPs with size 11 and 13 nm. The band appeared at 1683 cm−1 in PAA spectra which is assigned to C=O became doubling in the PAA blended samples due to the AuNPs may be coordinating with the oxygen atom in the C=O group. Fluorescence measurements indicated an emission band appeared at 585 and 590 nm for PAA doped by RhB and AuNPs with size 11 and 13 nm, respectively. The intensity of these emission bands was enhanced for the PAA blended by increasing both AuNPs and RhB ratios due to the interactions between dipole moments of the SPR of the AuNPs and RhB molecules. Therefore, it could be enhancing the fluorescence intensity of these samples, and it can be used as an laser active medium. Highlights Synthesis of AuNPs with different sizes. Prepare PAA blended by RhB and AuNPs. Study the coordination of AuNPs with PAA using FTIR. Explain the effect of RhB and AuNPs on fluorescence spectra.


Introduction
Noble metal nanoparticles have a particular position in the scientific community for their different optical properties, high surface to volume ratio, and surface plasmon resonance (SPR) [1]. SPR is a resonance obtained between the incident wavelengths and the collective oscillation of electron in the conduction band in the surface of metal nanoparticles (MNs), which is smaller than the incident light wavelength [2]. AuNPs have promising applications because of their physical, chemical, and fluorescent properties. These characteristics make it unanticipated applications in magnetic, electronics, and optical devices. Gold nanoparticles have received considerable attention from scientists because they are more stable and more application. Physical and chemical methods have been developed, some of which have been refined recently to fabricate metal nanostructures [3]. In the chemical method of metal NPs preparation, almost the synthesis parameters affect the size and shape, although the exact mechanisms involved in the size tuneability of noble metal NPs are still uncertain [4]. On the other hand, noble metal NPs were also synthesized using biological extracts due to costeffective and scalable [5,6]. Different methods of synthesizing AuNPs by reduction of Au 3+ ions using chemical reducing agents in solution have been reported [7]. Gold nanoparticles can be synthesized in various forms, including nanoshells, nanospheres, nanoprisms, and nanorods for a wide variety of applications. Murphy group [8] and Bastús et al. [9] prepared monodispersed gold nanoparticles in different size ranges (5-200 nm) diameter through reduction technique and controlled seeded growth through different reaction conditions as a solvent, reducing and stabilizing agent. Fluorescence occurs when the incident light excites the molecules; this makes molecules emit the light with the wavelength larger than incident wavelength [10]. The fluorescence of MNs could enhance the fluorescence of fluorophores when embedded dye with MNs, owing to the interactions among the dipole moments of the SPR of the MNs and the fluorophores of dye molecules [11]. Metal nanoparticles doped by polymers have been exploited for broad and different fields due to electrical and optical properties that depend on doped materials. Among several of these polymers, polyacrylic acid [12]. Moreover, polymers have an essential role in the metal nanoparticles doped by the dye, its works as a capping agent, in order to prevent the agglomerated of MNs during preparation [13]. Rhodamine B was used as a dye due to its fluorescent, it has photophysical properties, and excellent photo-stability [14].
In this work, we try to improve the fluorescence emission spectra of the polyacrylic acid blended by RhB (dye) and AuNPs with different sizes and shapes. Also, it is to understand how the addition of RhB and AuNPs in polyacrylic acid is effective in enhancing the fluorescence, due to the energy transfer from the fluorophores to the AuNPs.

Preparation of AuNPs
Gold salt was dissolved in distilled water with a molar ratio of (1 × 10 −4 M) as a stock solution. Trisodium citrate in concentrations of 1% was dissolved in distilled water. Gold nanoparticles were formed by preparing 11.31 ml of HAu Cl 4 (3H 2 O) with a molar ratio of (5.5 × 10 −3 M) and was added to 588.66 ml distilled water and heated under magnetic stirring, after 20 s of heating at 80°C. 0.0528 g of trisodium citrate was added to get concentrations of (3.24 × 10 −4 M) of TSC in a mixture. The colour of the solution was changed to red. Just as the colour changed, we pulled 75 ml of the mixture at 0, 1, 2, 3, 5, 7, 10, and 15 min, respectively.

Fabrication of PAA blended by RhB and AuNPs thin Films
Eight grams of PAA (450,000 g/mol) was added to 200 ml distilled water under magnetic stirring. The solution was heated until completely dissolved, at which 5 ml of dissolved PAA was poured on a petri dish (9 cm) to form a thin film. Rhodamine B (RhB) 0.074901 g was dissolved in 100 ml of distilled water under magnetic stirring at room temperature. To get an excellent thin film from PAA blended by RhB and AuNPs, 5 ml of dissolved PAA mixed with different ratios have an equivalent amount of RhB (5 × 10 −5 M) and AuNPs that prepared at 1 and 10 min and poured on a petri dish, respectively.

Samples characterization
The surface plasmon resonance of AuNPs colloidal was carried out using the UV-VIS-NIR spectrometer (Thermo-scientific Evolution 220 Spectrophotometer) at a resolution of 2 nm. Transmission electron microscope (TEM) was used to analyse sample morphology and to calculate the particle sizes. To prepare the samples for measuring;10 ml of the sample solution was placed on carbon-coated 200 mesh copper grids and allowed to air dry. TEM of the samples was performed using a JEOL JEM-1100 microscope (JEOL Ltd., Tokyo, Japan) equipped with a tungsten thermionic gun operating at a 100-kV accelerating voltage. TEM images were acquired by a CCD camera. The vibrational spectra were carried out using Jasco Model 300E Fourier Transform Infrared Spectrometer. The X-ray diffraction pattern was measured using CuKα radiation at 40 kV and 40 mA, and λ of 1.5406 Å. The scanning was performed over 2 θ range from 30°to 90°a t a speed of 0.02°/s. The crystal size of the nanoparticles was calculated using Scherrer's formula. Morphological characterization was carried out by using scanning electron microscopy (JSM-6380 LA). The fluorescence spectra were measured using the Jasco FP-777 spectrofluorometer. The light source is xenon arc lamp 150 W.

Ultraviolet-Visible spectra of AuNPs
The size and distribution of the nanoparticles can be comparable to the position and half-bandwidths of the surface plasmon resonance (SPR) band [15,16]. Figure 1 shows the UV-VIS spectra of AuNPs prepared at T = 80°C for different times. From this figure, absorption peaks were observed at different wavelengths in the range 516-540 nm for the samples prepared for 1, 2, 3, 5, 7, 10, and 15 min after the colour change to brown-red. These bands are attributed to surface plasmon resonance (SPR) bands. An SPR band is defined as a resonance influence generated a result of the interaction between the incident photons and electrons on the surface of AuNPs. Based on the work reported by Jayasmita [17], the localized surface plasmon resonance (LSPR) manifested because of the collective coherent electron oscillation at the surface of the Au nanoparticle, and an electric field was generated around the nanoparticle due to the photon interactions. The interaction between the incident photon and nanoparticles depends on their size and shape, as well as on the dielectric of the dispersion medium [18]. The surface plasmonic band was selected as a means of studying the optical applications of metal nanoparticles at visible wavelengths. As Figure 1 shows, the position of absorption band was 540, 530, 538, 528, 529, 525, 522 and 516 nm for the samples prepared for 1, 2, 3, 5, 7, 10, and 15 min, respectively. These spectra are dependent on preparation time and the size of AuNPs. The SPR bands changed to lower wavelengths with an increase in duration of heating at 80°C, as shown in Figure 2. The change to lower wavelengths (higher energy), known as a blue shift, indicates a decrease in the particle sizes of AuNPs [17]. In addition, the curvature of the nanoparticles can be affected by the SPR, such that at a very low surface curvature (a small particle size). The surface plasmon band shifted to lower wavelength and became narrower due to the orbitals being slightly farther from each other, resulting in a high energy resonance. Therefore, the blue shift was observed for small particle sizes. With the higher curvature in case of larger particle size, SPR shifted to higher wavelengths (a red shift), and surface plasmon bands became wider owing to the orbitals overlapping to a greater extent. Moreover, it was found that the half-bandwidth decreased with the decrease in particle size. The distribution of nanoparticles in solution reflects the number density. This number may be more affected than the volume fractions in cases of quantum size effect [19]. The particle size decrease with increasing heating times is probably due to the acceleration of the formation of Au +3 and the breaking of the bonds of the surface nanoparticles, which reduces the size of AuNPs. In other words, the increase in the heating time may increase the chemical reduction between tri-sodium citrate and Au + nanoparticles, which thickens the tri-sodium citrate shell and reduces the cores of AuNPs. Colour intensity and absorbance rates increased over the reaction time, suggesting an increase in the number of AuNPs and indicating a continuous reduction in gold ions. These results are consistent with a previous study by Ramakrishna [19]. Figure 3(a) shows the TEM image of AuNPs prepared at T = 80°C for 1 min, that mean the sample was taken in the first minute of AuNPs formation in solution. it can be seen that the gold nanoparticles have been observed and confirmed as black spot size, generally are spherical in shape, but sometimes are pyramid-, cylinderor character-L-shaped as well. Particle sizes have been determined by measuring the diameter of all spherical nanoparticles on TEM images and found to be around ∼ 11 nm. From the TEM image of AuNPs prepared at T = 80°C for 10 min, Figure 3(b) that mean the sample was taken in the tenth minutes of AuNPs formation in solution, it can be seen that the gold nanoparticle appears as a dark point with a spherical shape. The average size of the particle was estimated and found to be ∼ 13 nm.

FTIR spectroscopy of RhB, PAA, and PAA blended by RhB and AuNPs (with size ∼ 11 and ∼ 13 nm)
Figure 4(a) shows the transmittance spectrum of RhB. The band appeared at 3420 cm −1 is interpreted as the vibrations of the hydroxide group as a result of the sample's absorption of water molecules. The bands appear at 2960, and 2820 cm −1 are assigned to the symmetric and asymmetric stretching vibrations of CH bond  at 498 cm −1 is assigned to C = C-C [20]. Figure 4(b) shows the infrared spectrum of poly (acrylic acid) in the wavenumber range of 4000-400 cm −1 . The observed peaks in the 3700-3000 cm −1 range are suggested to indicate the stretching vibration (O-H) of the hydroxyl group, and the peak at 2944 cm −1 is assigned to symmetric or asymmetric stretching vibrations of C-H in the CH 2 group [21]. The broad low-intensity absorbance band observed at about 2645 cm −1 may be characteristic of the overtone and combination of the two chemical groups (C-O and C-H). The characteristic band at 1712 and 1683 cm −1 is interpreted as the C = O stretching vibration. The peak at 1436 cm −1 shows the vibrations of C-O-H group. The band appearing at 1243 cm −1 is explained as the coupled C-O and O-H in-plane bending vibration. The band noted at about 1197 cm −1 is attributed to the C-O-H group. The peak at 1045 cm −1 is a manifestation of the rocking bending vibrations of CH 2 , while the band at 919 cm −1 is attributed to the out-of-plane deformation of -COO-H group, and the one at 801 cm −1 indicates the stretching vibrations of C-COOH group. The peaks ranging from 900-400 cm −1 are complicated coupled vibrational modes, including the deformation (in-plane bending and rocking) of the CH 2 group. In addition, they represent the deformation (out-of-plane bending and in-plane bending) of the C = O and C-O groups, and the stretching of the C-C and C-O groups [21].
The absorbance spectrum of PAA was compared to that of PAA blended with two equivalent amounts of RhB and AuNPs (11 nm) at different molar ratios as shown in Figure 4(c). It was found that the absorption band appearing at 3424 cm −1 was shifted to a higher wavenumber in samples with Rh:AuNPs ratios (0.5:0.5), (1:1), and (2:2), respectively. This may be because the addition of AuNPs and RhB to PAA compresses the intermolecular bonding of PAA. The band noted at 3330 cm −1 disappeared in the spectrum of blended PAA. This is probably because the AuNPs had an electrostatic interaction with CO-OH. The band appearing at 2944 cm −1 in PAA was changed to lower and higher wavenumbers in the blended PAA as a result of increases in the CH group after the addition of RhB. The band at 1683 cm −1 became doubling in the blended PAA. This doubling was produced by the AuNPs coordinating with the oxygen atom in the C = O group. The band at 1243 cm −1 changed to 1297, 1288, 1286, and 1278 cm −1 in Rh:AuNPs ratios (0.5:0.5), (1:1), (2:2), and (2.5:2.5), respectively. These shifts may be owing to the change in C-O group caused by the addition of AuNPs and RhB. The absorbance spectrum of PAA blended by an equivalent amount of RhB and AuNPs (13 nm) with different molar ratios as cleared in Figure 4(d). It was shown that the absorption band appearing at 3424 cm −1 was shifted to a higher wavenumber (3461, 3477, 3470, and 3473 cm −1 ) in Rh:AuNPs ratios (0.5:0.5), (1:1), (2:2), and (3:3), respectively. This is because of the change in PAA structure resulting from the addition of AuNPs and RhB. The band at 3349 cm −1 was shifted to a higher wavenumber at (3373 and 3363 cm −1 ) in samples Rh:AuNPs ratios (0.5:0.5) and (1.5:1.5), and to a lower wavenumber (3266 and 3330 cm −1 ) in samples Rh:AuNPs ratios (2:2) and (3:3), respectively. The band appearing at 3212 cm −1 in PAA moved to a higher wavenumber (3247 and 3241 cm −1 ) in samples Rh:AuNPs ratios (1:1) and (1.5:1.5), and shifted to a lower wavenumber (3191, 3203, and 3166 cm −1 ) in samples Rh:AuNPs ratios (0.5:0.5), (2:2) and (3:3), respectively. This may be owing to the electrostatic interaction of AuNPs with CO-OH in blended PAA. The band appearing at 2944 cm −1 in PAA was changed to a lower wavenumber as a result of adding CH in RhB. The band at 1683 cm −1 is spletting and moved to a lower wavenumber, in my view because the AuNPs affected the electronegativity of the C = O group in the PAA. The band at 1243 cm −1 become broad and new band at 1220 cm −1 was observed. The band at 1197 cm −1 combined with 1243 cm −1 due to the change of structure during the addition of RhB and nanoparticles. This is possible on account of the electrostatic interaction between AuNPs and the oxygen atom in C-O. All these bands and their assignments were summarized in Table 1.

UV-Visible spectra of PAA blended by RhB and AuNPs (11 and 13 nm)
The polyacrylic acid film is transparent and has no absorption band in the wavelengths of the visible range. Figure 5(a) shows the absorption spectra of PAA film blended with an equivalent amount of RhB and AuNPs size (11 nm) and different molar ratios. It is known the AuNPs has an absorption band in the 540-516 nm range, and RhB has a band at 552 nm. Comparing PAA spectrum and PAA doping one, it was found that a new band appeared in the PAA doping at 560 nm and a shoulder at about 525 nm. This band and shoulder were assigned to the π-π* transition of RhB and surface plasmon band of AuNPs, respectively. The absorption band of RhB, formerly at 552 nm, was moved to the higher wavelength at 560 nm, and the surface plasmon band turned into the shoulder at 525 nm. This is perhaps due to the plasmon band overlapped with the RhB band so that the RhB band became broad and shifted to a higher wavelength. Likely, the interaction between trisodium citrate (capping of AuNPs) with RhB was responsible for this shift. The absorbance band intensity at 560 nm was increased with increases in the molar ratios of both AuNPs and RhB, and this increase follows the Beer-Lambert Law.
On the other hand, it observed that the halfbandwidth of π-π* at 560 nm decreases, and the absorption intensity increases at higher ratios of AuNPs and RhB. This increase is due to the increase in the colour intensity of the PAA film. Figure 5(b) shows the absorption spectra of PAA embedded with an equivalent amount of RhB and AuNPs (13 nm) with different Table 1. Infrared bands of rhodamine B, polyacrylic acid, polyacrylic acid blended with rhodamine B mixed gold nanoparticles (11 and 13 nm). molar ratios. The absorption band that appeared in the spectra at 563 nm, and the shoulder at 526 nm, were linked to RhB and AuNPs, respectively. By increased RhB and AuNPs, the intensity of the band at 563 nm was increased with increases in the two ratios of RhB and AuNPs. The half-bandwidth of π-π* electronic transition decreased with increases in the ratios of the embedded substances. This is because the AuNPs and RhB molecule's distribution was increased. From Figure 4(a, b), it can be noted that the half bandwidth has decreased with AuNPs (13 nm). This is perhaps because the nanoparticles became more stable. In addition, It can be noted a small variation in the π-π* band, which is probably owing to the increase in trisodium citrate and RhB around the AuNPs. Figure 6 shows the change in the X-ray diffraction pattern intensity with 2θ°in the 30°to 85°range of PAA blended with an equivalent amount of RhB and AuNPs of sizes (11 and 13 nm respectively. The diffraction pattern was similar to the Braggs' reflection of AuNPs. These results agreed with the data in JCPDS card no. 89-3697. These peaks confirm that the AuNPs exist as natural crystals. It can, therefore, be concluded that the RhB does not affect the AuNPs structures [22]. The crystal sizes (D) were estimated at each peak using Scherrer's formula [20],

X-ray diffraction of PAA blended with equivalent amounts of RhB and AuNPs sizes (11 and 13 nm)
where k is constant dependent of crystals shape (0.9), θ°is the Bragg angle, β is the full width half maximum (FWHM) in radians of the X-ray peak and λ is the X-ray wavelength (0.1540562 nm). The calculated crystal size was then compared with particle sizes measured from TEM images. It is worth noting that, the value of the nanoparticles calculated from the X-ray was greater than that of the particles found by TEM. This may be due to some agglomeration of AuNPs that occurred during the embedding AuNPs in the PAA. The difference between the average crystal size (at 24 nm) and all sizes (25, 22, 21, 25, and 18 nm) and (at 26 nm) and sizes (27, 25, 26, 26, and 19 nm) is because the average was calculated using the intensity of each peak. This means that the high-intensity peaks have more effect on the average size than the lower-intensity peak. By comparing the intensity of the peaks in AuNPs of sizes (11 nm) and that of the peaks in AuNPs of sizes (13 nm), it can be noted that there was an increase in intensity in the second set. This is probably due to the difference in nanoparticle sizes. Also, the broadening of the X-ray peak is inversely proportional to the size of the nanoparticles, as noted in the Scherrer equation results [20]. Figure 7(a, b) shows a scanning electron microscope (SEM) images of PAA blended with equivalent amounts of RhB and AuNPs (11 and 13 nm). From these two images, it can be noted that the nanoparticles appear as white spots with spherical or prism-like shapes. In scope, they appear in a random distribution with different sizes and shapes (see Figure 7(a)). It can be concluded that the gold nanoparticles of size (11 nm) mixed with RhB at the same molar ratio are randomly distributed in PAA film, and also of irregular size. They, too, appear as white spots with different shapes, as indicated in Figure 7(b). By comparing the SEM of PAA blended with the RhB and AuNPs with the TEM results of AuNPs, it can observe that the size of AuNPs increases in the SEM image. This is because some gold nanoparticles may have agglomerated during their embedding in the PAA. The fluorescence intensity bands that appeared at about 420 nm were weak compared to the fluorescence intensity bands observed at about 585.5 nm. This is maybe due to the weak excitement at the lower wavelengths. The emitted energy is less than the exciting energy, which may be owing to some loss of energy through heat or vibration in the excited molecules. The position of the emitted fluorescence depended on the difference in the dipole moments between the ground state (S0) and the first excited state (S1) of the RhB molecules. It can be noted that a redshift in wavelength, from 585 to 585.5 nm, from 585.5 to 588 nm, and from 588 to 589.5 nm, was observed from the lowest to the highest concentration of samples. A small linear increase was observed in the emission wavelength with an increased molar ratio of AuNPs and RhB due to the colour-bearing groups [1]. The observed peak and shoulder for the highest concentration of PAA blended with RhB and AuNPs occurred at longer wavelengths due to the existence of intermolecular interactions in the ground state in the PAA films and its doping. One of the most significant characteristics of the RhB and AuNPs-blended film is indicated by the Stokes shift formula, which describes the difference in the dipole moments between the ground state (S0) and the first excited state (S1) of the dye molecules. A measure of self-absorption of the fluorescent emitted rays can be calculated using the following relation [23].

Fluorescence spectra of PAA film blended with equivalent amounts of RhB and AuNPs (11 and 13 nm) with different molar ratios
where λ f and λ a are the wavelengths of the fluorescence and the absorbance maxima, respectively. The spectral features of the lasing RhB and AuNPsblended polymer depend on the intermolecular interaction between the RhB molecule and AuNPs with the macromolecules [11,[24][25][26]. In the presented study, it was found that the spectra of the blended polymer depend on the intermolecular interaction between AuNPs, RhB, and PAA. In addition, the Stokes shift values, as represented in Table 2, were increased by increasing the molar ratio of AuNPs and RhB, which indicates a low self-absorption of the fluorescent light emitted through the AuNPs and RhB molecules. From Figure 8(b) two fluorescence bands appeared: one at 582, 584, 585, 590, 590, and 588.5 nm, and the other at 402.5, 404.5, 410.5, 408.5, 409, and 405.5 nm. , respectively. The fluorescence intensity spectra increased with an increase in the ratio of RhB and AuNPs, as shown in inset Figure 8(b). The fluorescence intensity bands that appeared at about 404 nm were weak compared to the fluorescence seen at about 590 nm. This is because the 560 nm wavelength that is used in the excitation process has a weak effect on the surface plasmonic resonance for in RhB at the 350 nm level. It can be observed that the emission wavelength at 350 nm has no direct linear effect. At 590 nm, it was observed that increasing the concentrations of AuNPs and RhB increased colourbearing groups [1]. When the molar ratio of AuNPs and RhB blended in PAA was increased, a redshift in wavelength, from 582 to 584.5 nm, from 584.5 to 585 nm, and from 588 to 590 nm. This is due to the ground state absorption. The Stokes shift for this scenario can be calculated using equation 1, which listed in Table 2. The Stokes shift values in Table 2 were increased through an increase in the molar ratio of AuNPs and RhB, which can be described as a reduction in the self-absorption of the emitted fluorescence by the AuNPs and RhB molecules. In a comparison between the samples blended with AuNPs (11 nm) and AuNPs (13 nm), it was observed that the fluorescence intensity of the samples blended by (13 nm) was higher than the fluorescence intensity of (11 nm). This is maybe due to the increase in the lifetime of the excited electron at larger particle sizes.

Conclusion
The results of TEM confirmed the formation of AuNPs with various sizes and shapes, and the effect of preparation time on surface plasmon resonance (SPR) was clarified. FTIR results showed the bands appeared, at 1683 and 1243 cm −1 , were shifted after doping PAA by RhB and AuNPs due to the electrostatic interaction between AuNPs with PAA through C = O and C-O functional groups. A new band appeared at 560 nm for RhB and a shoulder at about 525 nm for AuNPs in visible spectra of PAA blended. Crystal sizes of the particle were calculated using the Scherrer equation from Xray results and compared with the particle sizes calculated from the TEM images. The crystal sizes for AuNPs blended in PAA are more significant than those calculated from the TEM images because of the aggregation of nanoparticles during their embedding in PAA. The fluorescence spectra reveal emission bands at 585 and 590 nm for AuNPs with size 11 and 13 nm, respectively. The higher bands are due to the electronic transition from surface plasmon resonance of the metal nanoparticles and π-π* of RhB to PAA. It can be inferred that gold nanoparticles and RhB blended in PAA film enhanced their fluorescence spectra of these samples. These meanes that these samples emit fluorescence with enhancing intensity and can be used as a dye laser medium.