3.1 The work environment
It is critically important to work in a particle-free environment when you’re working with nanoscale-sized particles. Therefore, test materials and calibration standards were prepared in a laboratory environment with a floor area of 780×960 cm2, satisfying the clean air requirements of ISO class 5 in operation. The classification has been performed using desecrate particle counter with a flow rate of 25 L/min. considering the particle size range of D≥0.3 and D≥0.5 µm. The standard preparation grinding and sampling processes were performed in this room. The scheme of the laboratory is shown in Fig. 1.
3. 2 Choice of indicator material
Phosphors are intensively used as raw material in production of light emitting diodes (LED) and other electronic materials including cathode ray tubes, field emission displays, and plasma display panels. Most of them are solid inorganic materials consisting of a host lattice, usually intentionally doped with a rare-earth or transition metal, or both of them. They emit light, or luminesces, when exposed to ultraviolet or visible light or an electron beam. The absorption of energy takes place via either the host lattice or on dopants and/or co-dopants taking place in the structure of the phosphor. In this work, commercially available Ce3+ activated Lutetium aluminum garnet (LuAG:Ce3+) was chosen as indicator material and has been subject of the quantification studies due to its bright luminescence, structural stability, particle size, and resistance towards air, water, pH variations, and, oxidizing and reducing agents, respectively. Fig. 2 reveals light-induced excitation and emission behavior of the Ce3+ activated LuAG powders. The two excitation maximum of the phosphor centered at 346 and 450 nm, arises from the 2F7/2 → 5d1 and 2F5/2 → 5d2 transitions of the Ce3+, respectively. The broad band emission peak centered at 540 nm can mainly be attributed to the back transitions of the excited electrons from the 5d to the 4f orbitals of the trivalent cerium [63,64]. In a similar way, the PMMA encapsulated LuAG exhibited a bright luminescence located at 512 nm when excited by the energy of the 450 nm of light (See Fig. 2). In this work, the intense emission of the LuAG:Ce3+ has been followed as the analytical signal to perform the quantification of nano-scale dusts in the air of the workplace, in form of aerosols. Since the selectivity of an analytical method is its ability to measure accurately the analyte in the presence of other potential interferences, the specific and concentration dependent signal observed at 512 nm, provided sufficient selectivity for the offered method. Herein, we used the certain concentrations of the LuAG:Ce3+ as standards for the calibration process as it has been convenient in spectral studies [65].
3.3 Grinding of the LuAG:Ce3+ particles
During grinding studies, 250 mg and 19 µm of phosphor samples were subjected to milling for three consecutive time periods of 120 minutes. Grinding balls were selected in appropriate number and diameter for the desired grinding size. For the first and second 120 minutes of grinding periods, 10 g of balls of 10 mm in diameter were used in each crucible. For further 120 minutes of grinding, 40 g of balls with a diameter of 2 mm was used. No lubricant is used for the balls and crucibles during grinding. Powders were added into the crucibles after precise weighing. Fig. 3 and 4 reveal SEM photographs of the phosphor particles before and after grinding, and, size distribution analysis results of the samples, respectively. At the end of the total grinding time of 360 minutes, the particle sizes were clustered in three groups; presenting average particle diameters of 233 ± 58 (13%), 1431 ± 440 (78%) and 5265 ± 434 nm (9%), respectively. The diversity in particle size and morphology observed in the SEM photographs is in accordance with the results depicted in size distribution analysis. Since the variation in particle sizes can also be seen in real working environments, the powders obtained by this way were subjected to transfer, washing, drying, weighing, and other processes without further grinding.
3. 4 Preparation of the electrospun fibers
The electrospinning technique was used as a simple way to fabricate the filter materials. Mid-molecular weight polymethylmethacrylte (PMMA) along with ionic liquid and the plasticizer was used to produce the electrospun fibers. The optimum precursor composition to form bead-free continuous fibers was provided by mixing 480 mg of polymethylmethacrylte (PMMA), 240 mg of plasticizer and 96 mg of ionic liquid in 6.0 mL of THF. After 8 h of stirring in the closed vial, the homogenous mixture was transferred into a 10 mL-plastic syringe. The solution flow rate was maintained at 0.5 mL/h by using the programmable syringe pump. An electric potential of 28 kV was applied between the needle of the plastic syringe and the aluminum substrate. Under the high tension, a Taylor cone; a jet of charged species; at the tip of the syringe body was formed, and the electrospinning took place. The resulting fibers collected on the substrate exhibited structural stability and integrity. The scanning electron microscope (SEM) images of the fibers under different magnification were shown in Fig. 5 Average fiber diameter was calculated by using at least 40 representative data points and reported as 657±78 nm. Presence of non-volatile room temperature ionic liquid, BMIMBF4 in the polymer provided excellent ionic conductivity during electrospinning. Although the imidazolium based ionic liquids have a reported absorption and emission in the visible side of the electromagnetic spectrum, the measured fluorescence of the BMIMBF4 was negligible in comparison to the strong emission of the dusts, and, did not suppress the indicating ability of the LuAG:Ce3+ in any way. The fabricated material was weighed on a precision balance and stored above the diffuser of the air pump. Fig. 5 shows SEM photographs of the fabricated electrospun fibers under different magnification before and after filtration process.
3. 5 Preparation of calibration standards
Calibration standards are critically important for the analytical process since they are used to draw the calibration curve with a high accuracy. In this study, the standards were prepared by using the high purity and strongly emitting LuAG particles at five different concentration points. The same composition offered for the electrospun fibers was used in the preparation of the matrix material of calibration standards. Amount of the phosphor for each calibration standard was adjusted as 1.0, 2.0, 3.0, 4.0 and 5.0 (±0.07) mg phosphor/kg polymer; respectively. The resulting composites were spread onto a 125 µm polyester support (Mylar TM type) with a knife-spreading device. Thicknesses of the films were measured using Tencor Alpha Step 500 Profilometer and the average film thickness was found to be 10.31µm (n=8). Each sensing film was cut to size 1.2×2.5 cm, fixed in the measurement cell of the spectrofluorometer, and the excitation/emission spectra were recorded for five different calibration points.
3.6 Calibration plot, limit of detection (LOD) and limit of quantification (LOQ)
In general, calibration of an instrumental method is very important and should be considered as the key point of the method validation. In the calibration process the use of correlation and/ determination coefficients as a test for linearity, the homoscedasticity of the experimental data, selection of appropriate weighting factor, and the regression, all are very important [66]. Herein the calibration plot was derived by using emission based response of the thin films. Fig. 6-I reveals excitation/emission spectra of the LuAG doped thin films acquired for five different concentration points upon excitation at 450 nm. The very strong and repeatable emission signal observed at 512 nm has been followed as the analytical signal. Emission intensities (dependent variable) were plotted versus corresponding phosphorus concentrations (independent variable) for five different concentration points. This study was repeated five times with five separate films for each calibration point. The resulting calibration plot derived using least squares method can be defined by the equation and R2 values of y=8×107x-6488 and R2=0.9799, respectively. Generally, a value of R2 greater than 0.990 is desirable. However, this is not the sole parameter in the evaluation of the linearity and is more realistic for the solution phase measurements where preparation of the calibration standards is easier than that of the solid state. Herein when the calibration standards and test numbers were considered, the obtained R2 value of 0.9799 looks like satisfactory for the solid state. The resulting calibration plot including error bars was shown in Fig. 6-II. The repeatability has been confirmed by using the 5 different graphical plots exhibiting standard deviations on the y axes for means less than 7.0%.
Limit of detection (LOD) has been approximated based on the standard deviation of the response (Sy) of the calibration curve and the slope of the calibration plot (S) according to the formula: LOD = 3.3(Sy/S). We determined the limit of quantification (LOQ) considering the lowest calibration standard on the calibration curve as the initial point where the detection response for the analyte was at least five times over the blank [66]. The upper limit of quantification (ULOQ) is estimated as the highest calibration standard on the calibration curve, where the analyte response was reproducible, and the precision and accuracy were within 15% of the coefficient of variation and 15% of the nominal concentration, respectively. Therefore, the LOD and LOQ were found to be 0.7 mg phosphor/Kg polymer and between 1.4-4.7, mg phosphor/Kg polymer, respectively.
3. 7 Stability
Many analytes may have a potential to readily decompose prior to measurement, during the preparation of the sample, transfer, extraction, or during storage. Therefore, it is necessary to clarify for the method how long a standard or sample can be stored before the analysis. Herein we tested spectral signal of the bare and PMMA embedded phosphors in air, water, in the presence of corrosive vapors of HCl, and in solutions of strong-oxidizing acid (2.0 of M HNO3) and strong base (2 M NaOH), after 12 h of exposure. While the bare phosphors stored in the alkaline solution exhibiting a signal drop of 5.0%, the PMMA embedded forms yielded almost the same emission intensity at 512 nm. Additionally, long-term photostability of the phosphor based thin films has been tested after 12 months’ storage in a desiccator in the Lab. environment. The observed signal drift was only 4.0 ±0.6 % (n=10) in direction of decrease in signal intensity, which can be concluded as an evidence of excellent short and long time stability the offered test materials.
3. 8 Processes, sampling and spectral measurements for real samples
The sampling of nano-scale particles from the air of the production lab. was performed via the vacuum pomp during the processes of weighing, grinding, emptying the crucibles and the final weighing. The grinding of each sample was performed at three different steps, with 10 mm diameter balls, at two consecutive speeds of 350 and 750 rpm, and, with 1mm diameter balls, at 750rpm, respectively. During the procedures, dusts in the ambient air were deposited on nano-sized PMMA filters for 3.0, 6.0 and 9.0 h of time periods. At the end of each sampling step, the electrospun filters were collected, dissolved in 6 mL of THF under magnetic stirring and the obtained homogenous mixture was used in preparation of thin films, as made for the calibration standards. Fig. 7 shows recorded excitation and emission spectra for the real samples after grinding durations of 120, 240 and 360 min. which corresponds to 3.0, 6.0 and 9.0 h of total processing time in the clean room along with other handling processes.
The spectral signals were computed via the calibration plot and converted to the airborne dust concentration for the samples. Table 2 reveals the recorded spectral counts (fluorescence intensities) and corresponding dust concentrations for the certain time ranges for two consecutive days. The presented emission based data and the calibration plot were the average of at least five replicate measurements. As can be seen from the table, the largest standard deviation of the counts and concentration measurements were less than 2.0 and 9.0 %, respectively. Recorded dust concentrations for the same time periods of the first and second days are in accordance with each other. Additionally, the calculated dust concentrations for the shortest time duration were above the LOD of the offered method.
Table 2. Recorded average spectral fluorescence intensities (n=5) and corresponding airborne dust concentrations. The sample collection was performed during grinding times of 120, 240 and 360 min., and following handling processes, for two consecutive day
|
1st day
|
Dust (mg dust/kg polymer)*10-3
|
2nd day
|
Dust (mg dust/kg polymer)*10-3
|
120 min. of grinding,3h of sampling
|
43900±927
|
0.6±0.010
|
59800±1069
|
0.8±0.009
|
240 min. of grinding 6h of sampling
|
126689±2497
|
1.8±0.012
|
125555±3027
|
1.7±0.011
|
360 min. of grinding,8h of sampling
|
139419±4875
|
1.9±0.017
|
159800±5127
|
2.1±0.015
|
For this work, the measured airborne dust concentrations from the laboratory environment falls within the range given in the calibration plot, allowing accurate measurement of the real samples without any mathematical transformation operation. In case of lower or higher sample concentrations, the extrapolation of the linear calibration plot is possible. For further deviations, the range of the calibration standards can be tuned to some extent considering the rules of the spectral measurements which allows application of the fluorescence based sensing approach to the quantification of other nano-scale luminescent materials.
Because exposure limits for other nanomaterials do not exist yet, herein we will compare our results and working range with the worker exposure limits of OSHA to nanoscale particles of TiO2 which recommends not exceed 0.3 milligrams per cubic meter (mg/m3) for 8 hours. The NIOSH’s recommended exposure limit for fine-sized TiO2 particles (particle size greater than 100 nm) is 2.4 mg/m3. The offered method also allows quantification of the dust concentration within these limits, without any conversion.