Different physicochemical approaches have been used by researchers to synthesize AgNP, these methods require the use of either strong or weak chemicals as reducing and protecting agents. Sodium citrate, sodium borohydride and alcohols are typical reducing and protecting agents [1] . However, the disadvantages of organic chemical as reducing agents are associated to their: low rate of synthesis, flammability and toxicity and effluent (a primary environment concern) [2] [3] [4] [5] . Hence, there is growing interest on alternate reduction agents benign to the environment, and plant extracts containing biomolecule are worthy and viable alternative reducing agent [6] . Kalishwaralal, Deepak, Ramkumarpandian, Nellaiah and Sangiliyandi [7] have used plant extracts to synthesizedhighly stable AgNP (40 nm), the biomolecular interaction with the silver ions contributed to the stable products. Nair and Pradeep [8] used Lactobacillusstrains with silver ions in the biosynthesis of nanoparticles and they alluded to the fact that if the lactic acid bacteria metabolites in whey protein are mixed with silver ion with results to the nucleation of the silver ions hence formation of AgNP.

Plants are known to chelate metals unlike microorganisms which are sensitive to metal ions. The ability of plant matrix to accumulate higher concentration of metal ions and the presence of biomolecules (polyphenols, flavonoids, etc.) enhance reduction, hence formation of particulates [9] . Bio-molecules provide a better reduction capacity compared to living organisms, hence synthesize a more stable AgNP [10] [11] . Hence, the method is referred as green and eco-friendly synthesis, it is low cost, eases in availability and much safer to handle [12] .

Many researchers have synthesis AgNP using medicinal plant extracts [13] [14] [15] . Kasthuri, Kathiravan & Rajendran [16] used the following medicinal plants (Oryza sativa, Helianthus annus, Saccharumofficinarum, Sorghum bicolour, Zea mays, Basella alba, Aloe vera Capsicum annuum, Magnolia kobus, Medicago sativa (Alfalfa), Ci namomumcamphora and Geranium sp.) extracts for bio-manufacturing of AgNP. Aloe vera plant is a shrubby, perennial, xerophytic, succulent, pea-green colored medicinal plant with fibrous or woody short stems covered by dense leaves partially buried in the soil. Aloe vera is a good candidate for the synthesis of nanoparticles because it contains functional bio-molecules (fat soluble photochemical such as flavonoids, organic acids and quinines). These chemical compounds found in Aloe vera are associated to the prevention of chronic and degenerative diseases [17] . They are known to neutralize reactive oxygen species by reducing the Ag⁺ ion to Ag⁰ during the synthesis of AgNP [18] [19] .

The rapid heating and penetration of microwave energy provide a significant advantage, because of the instantaneous and uniform energy distribution over the volume of the reaction, hence, shorten the chemical reaction period while the particle size distribution is enhanced [20] . Microwave heating is not new in the food industry and has various applications in the synthesis of different materials (benzyl chloride to benzyl alcohol, 1, 3-Dipolar cycloadditions of organic azides to ester or benzotriazolylcarbonyl activated acetylenic amides) [20] . Laser ablation is a process of optical amplification based on the stimulated emission of electromagnetic irradiation resulting to localized heating and photoionization [21] . Bauer, Abid, Ferman & Giault [22] used the laser oblation mechanism to excite AgNP in solution with ultrafast (femtosecond Picosecond and nanosecond) beam, which generates instantaneous relaxation of the hotelectrons, hence the Localized Surface Plasmon Resonance (LSPR) band resulted in the melting and eventual reduction of the targeted metal particles. It is therefore, hypothesized that laser ablation of the AgNP in solution will enhance the biomolecules such as aliphatic amines, alkenes (=C-H), alkanes (C-H), alcohol (O-H) and unsaturatedesters (C-O) in the Nano emulsion.

Fourier Transform Infrared (FTIR) has been used to determine concentration of bioactive compounds in plants extracts used as reducing agents of AgNP [23] . This technique has also been used by various researchers in the characterization of AgNP and AuNP and their associated biomolecules from plant extracts [4] [24] [25] . Hence, the objective of this study was to evaluate the effect of laser ablation on the physicochemical properties and stability of AgNP reduced with Aloe Vera (Aloe barbadensis).

The Aloe vera leaves were collected from the Alabama A&M University greenhouse in August, 2015 and were sterilized with ethanol to remove traces of soil, dirt and other debris. The parenchymatous (skin) of the leaves were separated from the gel by using an ethanol-sterilized surgical blade. The samples were ground with a coffee grinder to increase the surface area. Ten (10) grams of the ground Aloe vera skin material was placed in a 250 mL of Erlenmeyer flask and 50 mL ethanol was subsequently added and the extraction proceed overnight. The raffinate was filtered with a Whatman filterpaper (4 - 8 um).

The filtered Aloe Vera extract 3 mL was combined with 0.011 grams of silver nitrate and synthesized using a Microwave Work Station-240 with FISO Commander Workstation software for sensors and result management (FISO Technologies Inc. Quebec, Canada) and the pH of the Nanosolution was adjusted to 10 prior to the synthesis and was microwaved for 15 min. All the synthesis was conducted in a dark room to avoid Ag from absorbing light.

The particle size distribution was modified by exposing the Nanosolutions to the Nanosecond Pulse Laser (Surelite SL1-10 Seymour, CT, USA, λ = 1064 nm, P = 450 mL, 532 nm, P = 200 mJ) illumination treatments for 5 and 10 min. The laser illumination of 2 mL aliquots of aqueous colloidal solutions contained in a quartz cuvette (optical path length 1.0 cm, width 1.0 cm, height 2.0 cm) were conducted using the second and third harmonics setting of the laser according to the method described by Werner & Hashimoto [26] . Samples were collected and stored in the Food Engineering Laboratory walking refrigerator until needed for analysis.

The absorption spectra of the AgNP on solutions were characterized with a UV-Visible spectroscopic (Cary 3E UV-Vis, Varian PTY Ltd. Australia). The samples (AgNP synthesized in solution) 2 mL were placed in a quartz cuvette which was subsequently subjected to the UV spectroscopy to measure the absorption of colloidal suspension (hydrosol) and detection of the surface Plasmon resonance absorption peak.

Dynamic Light Scattering technique was used to analyze and quantify the particle size distribution of the AgNP. The Zetasizer Nano Series (ZEN 3690, Malvern Instruments Ltd, Worcestershire, UK) is the premium system in the Zetasizer range. AgNP synthesized were centrifuged to remove excess liquid prior performing the particle size and stability test. Three milliliter of the AgNP synthesized was placed in a quartz cuvette, and measurements were taken by intensity and volume according to the method developed by Kassama et al. [13] . The electrokinetic potentials (Zeta potential) of AgNP was also measured to determine the stability of the AgNP.

About 0.0016 grams of the treated samples were dropped on the FTIR card (Real Crystal IR Card, −9.5 mm Aperture, International Crystal Labs, New Jersey, USA) according to the method described by Chandran et al. [25] and allowed to dry. The cards were placed in the card slot section and the infrared light passed through, and continuing wave captured by the detector connected to a microcomputer (Thermo Fisher Scientific Smart Omni transmission, Madison, WI) hence displays the spectral profile corresponding to the different functional groups in the samples.

The experimental was a two-factor factorial design consisting of: Factor 1) Laser illumination with 3 levels of exposure time (0 min = T₈₂, 5 min = T₈₃, and 10 min = T₈₄); and Factor 2) Storage time with 3 levels of time (1, 3 & 6 weeks). The experimental data obtained was analyzed using SAS software (SAS v.8, Nashville). Mean comparison by Duncan’s multiple range test (DMRT) was used to separate treatment means (5% level of significance) for treatments that were significant by the Analysis of Variance (ANOVA). All experiments were carried out in triplicates and statistical tests were performed at 5% level of significance.

The AgNP nanosolutions were exposed to laser treatment for 0, 5, 10 min and stored for six weeks to evaluate its structural integrity and stability. The effect of laser treatment on the Surface Plasmon Resonance (SPR) spectral profile was monitored for each treatment and shown in Figure 1. The spectral characteristic absorption peaks at around 400 nm correspond to a surface plasmon band reported by Bae et al. [27] for AgNP. A surface plasmon peak shift to a slightly longer wavelength at a new peak of 430 nm was observed in Figure 1. The slight variation of the results could be attributed to the laser treatment, however the formation of AgNP at 430 nm is in line with values reported by Kassama et al. [13] [28] . The analysis of variances (ANOVA) of the results indicated that exposing AgNP solution to laser treatment significantly (p < 0.001) increase the concentration of nanoparticles. The intensity of the absorption peak was observed to increase significantly (p < 0.01) by 7% with 10 min laser exposure (Figure 1), similar observations was reported by Werner & Hashimoto [26] .

The surface plasmon peak shift which appears at the low-energy region could have influenced the 7% increase in nanoparticle concentration, because of the isolated particles that form densely packed aggregates through dipole interaction between the neighboring particles. Interestingly, this has manifested by the color variation of the nanosolutions during the laser oblation treatments. Moreover, the color intensity is due to the optical properties of AgNP which is strongly dependent on the particle size and shape. The optical properties were dominated by the collective oscillation of the conducting electrons resulting from the interaction with electromagnetic radiation during laser oblation [29] .

It was observed that the concentration and absorption increase with laser exposure time, thus the control (no treatment) had a concentration of 214 ppm while laser exposures for 5 min and 10 min produced a higher concentration of 224 and 229 ppm, respectively (Table 1). However, no significant change in concentration (p > 0.05) was observed during the six-week-storage period and no synergy (p > 0.05) was observed between the laser oblation and storage time. This is a unique physical attribute that denotes storage stability of laser treated AgNP.

The rate of reaction as a result of laser regeneration of the synthesis AgNP in solution is a zero-order reaction. Hence, a direct proportionality between AgNP concentrations and reaction time is shown in Figures 2(a)-(c). Although, few researchers reported the order of reaction in the biosynthesis of AgNP in the literature, yet Nair & Panda [30] reported the same order of reaction using Fusarium oxisporum to synthesize AgNP. The rate constants were calculated using the rate law, whereby the rate of disappearance of reactant is proportional to the concentrations of product formed. The rate constants for the different treatment exposure times, 0 min (no laser), 5 min and 10 min were 0.384, 0.408 and 0.4288 min⁻¹, respectively.

The results of the ANOVA showed that laser oblation significantly (p < 0.01) reduces particles sizes and thus impact the particles size distributions. However, no significant (p > 0.01) interactions were observed between laser oblation and storage time. The particle size distributions (PSD) of the laser treated AgNP solutions stored for weeks 1, 3 and 6 were used for the analysis. The results shows that about 90% of the volume of the measured colloidal AgNP in the solution with no laser treatment contains particle sizes with a hydrodynamic diameter of approximately 37.84 nm, in contrast with laser treated samples for 5 and 10 min were determined to be 10.1 and 8.72 nm, respectively. The AgNP solutions exposed to Laser oblation at 5 and 10 min showed a shift to the left of PSD curve, thus indicative of the effect on size reduction. The laser treatment impacted the bigger particles; hence their molecular bonds were broken hence reducing the sizes. A slight change in size distribution was observed after 5 min of laser exposure, however, Takami et al. [31] reported to obtain an optimum effect at 5 - 10 min laser exposure. Figures 3(a)-(c) show no changes (p > 0.05) in PSD