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Abstract
Barriers were produced in porous glass through its local bulk density modification by direct femtosecond writing accompanied by СО2-laser surface thermal densification, to make functional microfluidic elements separated by such physical barriers with different controlled permeability. The separation of multi-component solutions into individual components with different molecule sizes (molecular separation) was performed in this first integrated microfluidic device fabricated in porous glass. Its application in the environmental gas-phase analysis was demonstrated.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Porous materials are now intensively used as a host media for fabrication of integrated micro-analytical devices (IμADs), which are broadly used in biomedicine [1,2], genomics [3,4], virusology [1,5], and gas- and biosensing [6–9]. Such media hold the advantage of low reagent and analyte consumption. Finally, IμADs usually appear easy-to use, convenient and compact test devices for every-day applications.
Currently, cheap and easily manufactured IμADs are usually made of porous paper or polymer matrices [10,11] through polymerization [10], wax-printing [12], laser cutting [13]) to produce their internal impenetrable barriers (Fig. 1(а)). A set of such internal barriers arranged accordingly to predetermined embedded pattern enables to realize integrated devices in which each cell is filled by specific reagent and can function independently. Meanwhile, soft material platforms (polymers, paper, etc.) for IμAD fabrication possess intrinsic physical and chemical shortcomings:
Fig. 1 Schematic view of integrated fluidic device with following impregnation with several dyes (a), where the barrier supports isolation of individual components in cell. Laser-based methods for porous glass density modification: inner elements of a barrier by femtosecond laser densification (b); surface densification by CO2-laser action (adapted from [14]).
- 1) Common porous host matrices are not physically (mechanically and thermally) and chemically stable, as a result of which the service life and conditions of their operation are substantially limited and therefore they are declared to be single-use.
- 2) For the same reason usual porous host matrices are not chemically inert, influencing the analytical results and their interpretation.
- 3) Porous host matrices are desired to be optically transparent medium for their optical operation and control, to provide their increased integration level [15].
In this lieu, we propose nanoporous oxide alkaline boron silicate glass (PG) [16] as a novel matrix for IμAD, with its high absorption capability for capturing reagents and analytes [17,18], and its nanoscale, essentially deep sub-wavelength pore size, enabling its high optical transmission (~90% in the visible and near-IR range of wavelengths). Physical and chemical properties of PG are comparable to fused silica, in such a way the pore structure remains constant even at temperature close to 700 °C. A prevailing amount of SiO2 in the composition (> 95%) significantly increases the mechanical and chemical strength of the matrices and excludes its influence on the result of the analysis. Due to the possibility to vary its pore size in the wide range of 2-70 nm [19], PG is a promising platform for the development of nanocomposite materials with desired properties by its impregnation with different substances (nanoparticles of metals, semiconductors, ferroelectrics, liquid crystals) [20,21] or organic components for gas analysis and other analytical applications [22–24]. However, so far fabrication of integrated micro-devices on the PG platform was not possible at the absence of any technology, enabling local bulk variation of PG density to make partially or completely impermeable internal barriers.
Up to now, this technology was developed only for femtosecond laser fabrication of waveguide-like structures [25] (Fig. 1(b)) or near-surface PG modification via strong СО2-laser absorption and the resulting near-surface local thermal densification by collapsing nanopores in the laser-heated porous matrix at very shallow depths [26,27] (Fig. 1(c)). In this study, microscopic cellular structures were fabricated inside a 1-mm thick PG plate in the form of separating walls, penetrating all its depth and presenting the barrier with controlled permeability regarding to organic dyes. For this purpose, direct laser writing was used in multi-pulse irradiation regime to form at the required PG depth a monolithic pattern of densified tracks without residual stresses, by means of tightly-focused femtosecond laser pulses at 515-nm wavelength, at which this material is transparent. Since this bulk densification mechanism was not operative on the PG surface, the wall fabrication over the PG plate surface was accomplished by means of thermal СО2-laser densification. Different tests of cellular structures were performed to confirm its permeability and sensing capabilities.
2. Materials and methods
2.1 Porous glass samples
1-mm thick PG plates for our experiments were prepared, using phase-separated 8V-NT glass [28]. The initial two-phase glass was synthesized in the factory by the charge melting method via the technology of manufacturing optical glass. Their chemical composition was (mass fraction, %): 0.30 Na2O – 3.14 B2O3 – 96.45 SiO2 – 0.11 Al2O3. The average pore radius was 3.5 nm at the porosity of 26% and the specific pore surface of 210 m2/g [19]. These PG plates possessed high transparency (~90%) in the visible and near IR ranges (0.2 – 2.5 μm), with the average refraction index n ≈1.342 in the visible spectral range.
2.2 Selective bulk and surface laser densification
The fabrication of internal barriers in the form of embedded arrays of densified PG micropixels was performed in several steps (Figs. 2(a) and 2(b)). At the first step, successive scanning in the PG bulk was performed, using 200-fs (full width at half maximum), 515-nm femtosecond laser pulses with the maximum pulse energy (Ep) of 4.45 μJ (TEM00 mode), delivered by an Yb-doped fiber laser Satsuma (Amplitude Systèmes, France) at the repetition rate of 500 kHz (Fig. 2(a)). A silica-glass microscope objective (10Х, 0.25) focused the laser radiation into the focal spot of 2ω0 ∼2.8 μm in 1/e-diameter, providing the peak laser intensity of 8·1013 W/cm2 at Ep = 2.0 μJ. Hence, to avoid overexposure of the already present tracks because of the extended focal length.
Fig. 2 Two-step approach to form a through barrier inside the PG plate: (a) femtosecond laser inner densification (λ = 515 nm, τ = 200 fs, Ep = 1.8 μJ) according to the template ((b) - side view of the barrier); (c) PG surface sintering by CO2–laser (λ = 10.6 μm, τ = 190 μs, q = 2·104 W/cm2) to complete the barrier. (d) Final view of 4 independent cells formed in the PG plate. SEM insertion image of the nanoporous glass structure.
The template of the barrier with selective permeability presented in Fig. 2(b), requires scanning of the focused laser beam for each line along X-axis (track length) with the variable displacement along Z-axis (track height). Simultaneously, the laser beam should be slightly shifted from the line to the line along Y-axis by steps of variable micro-scale width p, exceeding the waist diameter 2ω0 ∼2.8 μm to avoid overexposure of the previous fabricated tracks. Moreover, complete impermeability was achieved, keeping the magnitude p ≥ 2ω0 (effectively, about 3 μm). The variable incident laser exposure enabled to produce inner densified structures with their heights and widths p, varying from 10 μm up to 100 μm and 3-6 μm, respectively. As an example, ten 100-μm high tracks made the barrier across the 1-mm thick PG sample.
Finally, the mechanism of the fs-laser induced bulk densification was found to be not operative during densification of the PG surface - the final step in fabricating the hermetic barrier (Fig. 2(с)). The reason of that is the difference between surface densification and surface ablation thresholds are relatively small at femtoseconds pulse duration and there is not possible to find a stable regime for surface densification. As a result, this procedure was accomplished by means of a single-mode CO2-laser irradiation [14,26] (Synrad 48-1(S)W, periodic-pulse regime, 10.6-μm wavelength, pulse width τ = 190 μs, repetition rate ν = 5 kHz, beam quality M2 < 1.2), focused by a ZnSe lens with a focal length ~70 mm. Then, 50-μm wide densification tracks on the PG surface were formed at the power density q = 2·104 W/cm2 by scanning the laser beam at the speed of 0.2 mm/s. The combination of these steps enabled fabrication in the PG plate of a few liquid cells separated by the barriers (Fig. 2(d)).
2.3 Evaluation of barrier permeability
Permeability tests for the fabricated barrier in the PG were performed as follows: the edge of the plate was immersed into a coloured liquid dye solution and remained there until the cell was completely filled in by the impregnating fluid (Fig. 3(a)).
Fig. 3 Tests of the barrier permeability: (a) schematic view. Impregnation process: (b) 1 s, (c) 450 s.
The colorizing test fluids were water solutions of rhodamine 6G (with the molecular size ~1.8 nm [29]), fuchsine (~1.2 nm [30]), bromocresol purple and thymol blue. The impregnation process was monitored by means of an optical microscope equipped by a video camera (Figs. 3(b) and 3(c)). Upon the complete filling of the cell, the sample was dried on the filter paper for 30 minutes to verify the cell hermeticity. Then, spectral transmittance characterization was performed by means of a microscope-spectrophotometer (MSFU-K Yu-30.54.072) in the range of 350 – 900 nm (size of the characterized region was equal to 5 μm).
3. Experimental results and discussion
3.1 Laser procedure
Densified waveguide-like tracks with the increased refractive index were fabricated inside PG at Ep > 1.5 μJ and 400 – 4000 laser pulses per spot (Fig. 4(a)). Their optical characterization in the linearly polarized light (with the crossed polarizer/analyzer) indicates the absence of cracks and residual mechanical stresses (Fig. 4(b)). The observed local densification of the PG network within the laser beam waist apparently proceeded via heat accumulation from multiple ultrashort laser pulses, enabling the local temperature to reach thousands of Celsius degree [31,32], inducing the local pore collapse. In contrast, the increasing number of the laser pulses (>4000) and their energy Ep (> 2.2 μJ) resulted in decompaction of the PG structure in the form of the dark spherical foam-like regions (Fig. 4(c)). Cross-sectional TEM characterization of these diverse tracks is currently in progress.
Fig. 4 Regimes of structural modification inside the PG plate by femtosecond laser pulses presented in the form of energy in the pulse vs the number of laser pulses per spot. Optical images of the densification track formed at Еp = 1.8 μJ and N = 1000 (а, b) and decompaction track formed at Еp = 2.0 μJ, N = 5000 (c, d). These images of densification and decompaction areas were made in linearly polarized light (b, d).
In comparison with the densified tracks, the lateral size of such decompaction tracks increased two-fold, being accompanied by residual lateral stresses (Fig. 4(d)). The origin of decompaction lies in overheating of the glass material in the focal region by the high-energy fs-laser pulses, coming at the high repetition rate (the local heat source). This local heating is evaluated to give rise to local temperatures up to 3·103 °C [33], exceeding the temperature of silicon dioxide decomposition of ~2.2·103 °С [34].
Although local PG densification in the form of the densified micro-track arrays across the PG plate allowed the fabrication of different embedded barriers, there is still a problem with open pores on the PG surface. Its local surface sintering was accomplished by СО2-laser irradiation (Fig. 5(a)) due to its strong absorption by the silica glass network and subsequent heating till the temperature of viscous flow of glass (~900 – 1400 °С) [35,36]. The width of the sintered region on the PG surface, equal to the bulk barrier width within 5%, was investigated optically (Fig. 5(b)) and by scanning electronic microscopy (SEM Carl Zeiss Merlin) (Fig. 5(c)).
Fig. 5 Top-view SEM image of the CO2-laser surface-sintered barrier in the PG plate (a). Cross-sectional SEM image of the sintered surface region (b) with its magnified view in (c).
3.2 Pre-testing of barriers penetrability
Cellular structures were fabricated inside the PG plates by fs-laser writing of buried arrays of the densified tracks with variable period p. Variable p provided the variable barrier permeability changed, enabling separation of molecules of different size or mass. For example, the close contact between the densified tracks (p ≤ 5 μm), the final barrier provided the separation of molecules with higher mass. As a result, such barrier was impermeable for dye molecules, but small molecules of water could easily leak through (Fig. 6(а)) during impregnation with water solution of rhodamine 6G. This property of partial barrier penetrability can be applied for molecule separation in different medical applications and molecular biology [37,38]. The increase of the period (p ≥ 5 μm) results in penetration of the dye molecules through the barrier (Fig. 6(b)).
Fig. 6 Testing of the barriers with full (a) and partial (b) impermeability during PG impregnation by water solutions of rhodamine 6G and fuchsine. The scale bar is 100 μm.
The sample transmittance spectra were acquired across the PG plate (Fig. 7) to test the permeability of the fabricated barriers regarding the various dye molecules. The initial PG plate, which possesses high optical transmittance (~75%) in the spectral range of 350-900 nm (red square) was taken as the reference. The rhodamine molecules in the glass pores result in the modified transmittance spectrum, exhibiting the intense absorption in the blue-green spectral range (400-550 nm) and in the near-IR range (> 850 nm) (blue triangle). The filling of the glass pores and channels with water molecules results in the very minor variation of the transmission (green circle) regarding the initial PG one.
Fig. 7 Transmission spectra of 5-μm wide regions in the PG before (coloured by the impregnating rhodamine 6G water solution) and beyond (transparent water only) the barrier, with the dry PG region as the reference. The scale bar is 20 μm.
Since the barrier permeability is managed by the period p between the densified tracks, it is natural to study the relationship between masses of penetrating molecules and the track period. In this lieu, we suggest an Arrhenius-like expression to fit our experimental results presented in Fig. 8:
where Mmax – maximum mass of a molecule, which can penetrate into PG with the pore size of 3.5 nm; D – pore size; с – calibration coefficient; p – period between densified tracks.Fig. 8 Experimental diagram of the barrier permeability in the coordinates «molecular mass – track period». The exponential curve represents the relationship between the molecular masses and the track period, dividing the diagram into the coloured “permeability” and “impermeability” regions with the numbered circles representing the utilized dye molecules and water. The overlapping of the tracks for p < 3 µm results in the heavily damaged barrier (“barrier destruction”).
The proposed functional dependence M(p) in its lower limit tends to the period of track ≤3 µm (the laser beam waist diameter), which, as mentioned above, makes the barrier completely impermeable even for water molecules (the “salmon” colored region in Fig. 8). Moreover, fs-laser writing with the track periods close to 3 μm can break the barrier due to accumulation of mechanical stresses which value can overcome the destructive value.
3.3 Analytical cells for environmental analysis
One of possible applications of barriers and individual cells built by such barriers is the fabrication of integrated gas indicator. This implies presence of several cells integrated into one PG plate and filled with different reagents, which can operate independently from each other. As an example, a number of dyes can fill the cells inside the PG plate to react on ambient gas composition.
In this work, the PG plate was divided by one barrier into two cells (Fig. 9(а)), impregnated for 10 minutes by blue thymol [39] and bromocresol purple [40] dyes, respectively. The blue thymol dye reacts with carbon dioxide (CO2), if the relative gas content in the atmosphere exceeds 0.1%, changing the colour from blue to yellowish via protonation (change of pH from 9.6 to 8.0) [41]. The other bromocresol purple dye reacts with ammonia vapour (NH3), changing its colour from yellow to blue (change of pH from 5.2 to 6.8) [41], due to the loss of two protons. Thus, we use these two dyes opposite in their acidity to have the indicator operable both in acid and alkaline media. The fabricated barrier impedes the mutual penetration and mixing of the dye molecules (Fig. 9(b)). Before cells testing, the transmission spectra of their raw dye forms were taken (Fig. 9(d)).
Fig. 9 Gas sensing by fluidic cells: two fluidic cells in the PG plate separated by the barrier (a). Impregnation of these cells with thymol blue and bromocresol purple (b). Photometric gas sensing of carbon dioxide in the cell with thymol blue and ammonia gas in the cell with bromocresol purple (с), illustrated by transmission spectra measured in each cell before and after gas interaction (d, e).
During the environmental gas analysis, the PG-based indicator was placed inside a 5-liter chamber, filled by CO2 and NH3 gases at the pressure of 1 atm, for each gas to occupy ≈5% of the chamber volume. The reaction time was about 5 min, sufficient for noticeable re-coloration of the analytical cells (Fig. 9(с)), then the indicator was removed for the optical transmission spectroscopy in each cell (Fig. 9(d,e)). In the cell with blue thymol, upon the reaction with СО2 its transmittance over the range of 600 - 800 nm increases by more than 3 times (Fig. 9(d)). Similarly, during the reaction between bromocresol purple and ammonia vapour there is a drastic reduction of the transmittance in the yellow range (550-600 nm) and an increase of the transmission in the violet range (400-450 nm). This is in agreement with the result obtained previously [40].
The distinctive feature of PG application is the possibility of repeated use of cells with barriers by cleaning of impregnated cell by heat treatment in the furnace. For the cells cleaning from the remainders of the previous substances the temperature of 600 °С was chosen and this temperature was enough for the removal of organic compositions from the glass pores [42]. The degree of PG plate purification from the dyes was confirmed by transmission spectra restoration to the initial values, taking into account the measurement error of 2%. Thus we showed the possibility of repeated thermal treatment of the sample without destruction of the barriers integrity and total cells purification from the dye.
4. Conclusion
In this work the nanoporous glass plate was for the first time utilized as a host material platform for direct femtosecond laser writing of controlled embedded micro-scale barriers in its bulk in the form of densified tracks. Within this fabrication technology, the barrier permeability is manageable by varying the track period in the range of 4-5 µm, while barrier sets make individual cells (100 cells of 1x1 mm size per a porous glass plate of the standard size of 1х1 cm) with their specific permeability. When filled by a colour dye indicator, these cells were demonstrated to enable environmental gas analysis, working as ultra-compact integrated gas indicators.
Funding
Ministry of Education and Science of the Russian Federation 14.578.21.0197 (RFMEFI57816X0197).
Acknowledgment
The reported study was financially supported by the Ministry of Education and Science of the Russian Federation, research agreement No. 14.578.21.0197 (RFMEFI57816X0197).
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