Silica-Based Organic-Inorganic Hybrid Fluorescent Ink for Security Applications.

A facile formulation of fast-drying fluorescent ink made from nanostructured fluorescent silica nanocrystals is presented. The rheostable viscous ink suitable for screen printing was developed by careful selection of organic vehicle components, which was later printed onto various rigid and flexible substrates. Photoluminescence studies of the printed film confirmed that the formulated ink composition did not show noticeable influence on the excitation property of the fluorescent silica. The developed cost-effective and fast-curing fluorescent silica ink with desirable luminescent property makes it a suitable candidate for information encryption, optical devices, and energy conversion applications.

Introduction

In recent years, fluorescent nanoparticles have attracted much attention in emergent fields such as nanobiotechnology, photonics, and optoelectronics.[1,2] Commonly used fluorescent materials are organically modified silica, hydrophobic and hydrophilic organic polymers, semiconducting organic polymers, quantum dots, carbonaceous nanomaterials such as carbon dots, carbon nanoclusters, and nanotubes, metal particles, and metal oxides.[3,4] However, when considering limitations such as low detection efficiency, complicated processing, and toxicity, hybrid structures formed of an organic dye molecule-incorporated inorganic matrix are highly recommended.[5] This core–shell architecture has an added benefit of designing new geometries by incorporating multiple functionalities into a single nanoparticle.[6,7] Of various inorganic matrixes, the silica matrix has been widely studied because of its biocompatibility nature. Moreover, silica can act as a mechanically and chemically stable vehicle, thereby protecting the encapsulated dye from external perturbations.[8,9] Fluorescent silica nanoparticles have pertinent applications in the field of information technology, biotechnology, and medicine owing to their tunable size, optical transparency, high hydrophilicity, biocompatibility, and low cytotoxicity.[10] Recently, fluorescent inks have also been used as security markers with the objective of preventing theft and forgery.[11,12]

Recent advances have been focussed on organic fluorophore-encapsulated hybrid structures because of their increased brightness and photostable matrix compared to the single organic dye. Incorporation of dyes into the silica matrix can be easily achieved with either covalent bonding[13] or noncovalent bonding[14] of the dye with the silica matrix. Considering several advantages, the noncovalent approach is the most preferable way to encapsulate the fluorophores inside the sol–gel-derived colloidal silica matrix.[15] The well-known Stöber method was traditionally chosen for sol–gel synthesis of monodisperse nanomicrometer-sized silica particles. In such a typical process, the molecular precursor of silica is initially reacted with water in an alcoholic solution and then the resultant molecules are assembled together to build larger complex structures.[16]

Several modifications were carried out to achieve size-controlled synthesis of silica nanoparticles.[17,18] In 2005, Ow et al. successfully synthesized nanometer-sized fluorescent silica nanoparticles through a modified Stöber route, in which a core–shell architecture consisting of a fluorophore-rich center was capped within a siliceous shell.[19] With the aid of a similar core–shell architecture, silica nanoparticles incorporated with fluorescent dyes were also developed subsequently by Burns et al. for chemical sensor applications.[20] In an interesting work published in 2013, Yoo and Pak synthesized size-controlled fluorescent silica nanoparticles through a reverse (water in oil) microemulsion method.[21] Because the dye-doped silica nanoparticles are susceptible to leakage and irreversible photodestruction when dispersed in organic solvents, controlling their dye leakage is highly crucial in security applications. In the present study, a double-layer approach was developed to reduce the dye leakage, wherein the fluorophore molecules are encapsulated in the silica matrix. Here, the second outermost silica layer, created by the condensation of alkoxy silane, will successfully incorporate the organic fluorophores into the silica double layer through electrostatic attraction.

For practical printed tag applications, screen printing is a widely adopted strategy. Development of a colloidal fluorescent ink is a challenge because the ink’s vehicle contains dispersants, solvents, and binders, whose presence can eventually lead to photodegradation. In general, fluorescent inks provide a platform for a wide range of applications in domains such as information storage, bioimaging, smart packaging, and nanoelectronics.[22] For the past few years, carbon dots and functionalized carbon dots, and coumarin derivative fluorescent ink were extensively studied.[23] In 2012, low toxic and biocompatible carbon dot-based fluorescent ink was formulated and its distinct fluorescent properties were studied.[24] Gao et al. reported green route synthesis of nitrogen-doped carbon dots, and their performance as fluorescent ink and electrocatalysts were demonstrated.[25] Metal-free water-soluble graphitic carbon nitride quantum dots as invisible security ink was developed by Song et al. for data security applications.[24] Recently, nitrogen-doped graphene dots and magnesium–nitrogen-embedded carbon dots were reported for anticounterfeiting and copper metal sensing applications, respectively.[26,27] For anticounterfeiting applications, inkjet printing of colloidal photonic crystals was extensively studied.[28−30]

So far, no serious attempts were made toward formulating leak-free core–shell based fluorescent silica - ink having stable luminescence  and low toxicity. Herein, we report the formulation of a stable screen-printable fluorescent ink by choosing fluorescent double-layered silica nanoparticles as the filler, Butvar as the binder, and sodium dodecyl sulfate (SDS) as the dispersant. The presently adopted core–shell architecture-based ink is advantageous because it prevents cluster formation. Owing to its stable fluorescence and good solubility in ethanol medium, our quick-dry fluorescent ink can find applications in the field of printable fluorescent tags.

Materials and Methods

Tetraethyl orthosilicate (TEOS, 99%), 3-(aminopropyl)triethoxysilane (APTS, 98%), fluorescein (free acid, C12H20O5), ammonium hydroxide (NH4OH, 28–30 wt % ammonia), cyclohexane (C6H12, 99%), 1-hexanol (C6H5OH, 98%), poly vinyl butyral (PVB) (registered trademark of Solutia, Inc. as Butvar), and SDS were purchased from Aldrich Chemicals Co. Brij35 [a polymer of ethylene glycol and dodecyl ether ((C2H4O)23C12H25OH)] was received from Merck Chemicals. All the raw materials and solvents were used as received without any further purification.

Synthesis of Fluorescent Double-Layered Silica Nanoparticles

Fluorescent double-layered silica nanoparticles were synthesized in a reverse microemulsion method.[21] A reverse microemulsion (water in oil, W/O) is an isotropic and thermodynamically stable single-phase system that consists of water, oil, and surfactant, which is highly stable than normal emulsion method. The water droplets were first stabilized by surfactant molecules and then get dispersed in the bulk oil phase, which serves as reactors for the synthesis of nanoparticles. In brief, Brij35 (5.4 g, 1.6 mmol) was added with 38.5 mL of cyclohexane and 8 mL of 1-hexanol, and the solution was kept under sonication until a transparent clear solution was obtained. Deionized water (1.7 mL) was added and sonicated further for 15 min. To the clear solution, 0.5 mmol of TEOS was added and stirred for 30 min. Prior to hydrolysis and condensation of TEOS, aqueous fluorescein (0.4 mL of 1 × 10–2 M, 1.6 μmol) was mixed to the reverse microemulsion. In the microemulsion, TEOS was initially hydrolyzed by adding ammonia and later condensed in the presence of fluorescein to generate fluorescein-doped silica nanoparticles. Without breaking the microemulsion system, APTS (0.5 mmol) was then added into the above reaction mixture before removing excess fluorescein, where APTS has been hydrolyzed and condensed on the surface of the fluorescein-doped silica nanoparticles. This sequential process helped the formation of the second layer of silica and finally yielded the core–shell structured double-layered silica nanoparticles. The synthesized double-layered silica nanoparticles have good solubility in aqueous medium and were separated from ethanol by centrifugation. The microemulsion was then destabilized by adding acetone, centrifuged, and washed with ethanol four to five times to obtain fluorescent double-layered silica nanoparticles.

Formulation and Screen Printing of Fluorescent Silica Ink

A stable dispersed fluorescent ink can be developed by judiciously blending the double-layered silica nanoparticles with a suitable surfactant, binder, and carrier solvent.[31] In the present work, 60 wt % fluorescent silica, PVB (Butvar), and SDS and ethanol were chosen as the filler, binder, and surfactant, respectively. Here, the ink formulation was begun with mixing of 2 wt % dispersant (w.r.t filler) with ethanol in order to enhance the dispersion process. The process was carried out for 10 min on a magnetic stirrer. Later on, fluorescent silica nanoparticles were added and stirred continuously for another 6 h, followed by the addition of the binder. The whole solution was stirred again for 24 h to derive a homogeneous viscous ink. The ink was screen-printed with the desired pattern with the aid of a screen mesh having 350 counts. The customized screen mesh in the present study was created by coating the screen in photoemulsion and curing the areas which are required, and the surplus emulsion was washed off to obtain the desired pattern.[32]

Characterization

The nanoparticles and the core–shell structure were analyzed by high-resolution transmission electron microscopy (HRTEM) (FEI Tecnai G2 30S-TWIN, FEI Company, Hillsboro, OR). The rheological characteristics of the ink were measured using a Rheo plus32 rheometer (Anton Paar, Ashland, VA, USA) operating in both rotational and oscillation modes. Screen printing was done through semiautomatic screen printer (EKRA, Germany) which operated at a printing speed of ∼30 cm/s. The ink was screen-printed on various substrates, and the surface morphology was analyzed by using a scanning electron microscope (Zeiss EVO 18 Cryo SEM, Jena, Germany). The surface roughness of the printed film was analyzed by atomic force microscopy (AFM) (MultiMode, Bruker, Germany). UV–vis spectra of the solution state were recorded with a UV spectrophotometer (UV-1800 Shimadzu) in the range of 200–900 nm. The absorption properties of the printed pattern were characterized using a UV–vis near-infrared source (Ocean Optics). Excitation and emission spectra of the solution and film states were recorded with a Spex-Fluorolog FL22 spectrofluorimeter equipped with a double grating 0.22 m Spex 1680 monochromator and a 450 W Xe lamp as the excitation source.

Results and Discussion

Microstructural Analysis of Fluorescent Double-Layered Silica Nanoparticles

As described earlier, the fluorescent double-layered silica nanoparticles were developed by the coupling of fluorescein species as a core in core–shell architecture. One of the principal merits of this procedure is that the size of the silica nanoparticles can be effectively tailored by changing the water-to-surfactant ratio. These cores can act as nuclei for the growth of the silica shell, thereby protecting the encapsulated dyes.[19] For a screen-printable stable dispersed ink, uniform particle size distribution is more preferred because of microstructural homogeneity considerations. Prior to the ink formulation, particle size and morphology analyses were carried out by TEM. Sample preparation was done by suspending double-layered silica nanoparticles in ethanol medium, and the particle size and morphology were studied. TEM analysis of the abovementioned monodispersed solution was carried out by drop-casting it onto a copper grid, and the microscopic images are depicted in Figure . TEM images clearly show the spherical morphology of the fluorescent nanoparticles, and the average particle size of double-layered silica nanoparticles comes around 70–80 nm, which is in the desirable range to maintain the colloidal stability of the ink suspension.[33] Thin mesoporous shells are uniformly coated over the silica core surface at a thickness of ∼15 nm. The peculiar architecture effectively protects the fluorescein dye molecules within the matrix and was developed in a uniform and orderly fashion.

Figure 1

(a–c) TEM micrographs of fluorescent silica nanoparticles in ethanol medium, under different magnifications.

The security features inscribed using invisible fluorescent inks have widespread applications in passports, bank notes, and credit cards. Maintaining the fluorescence intensity in the printed ink is a challenge because the ink contains other organic components such as solvents, dispersants, binders, and homogenizers. The formation of double-layered silica nanoparticles with encapsulated dye molecules and their journey into a flatbed printed structure are schematically shown in Figure .

Figure 2

Schematic description of the synthesis of screen-printable fluorescent double-layered silica ink.

Ink printability is a function of both physical and chemical properties of the filler, additives, and solvents employed in the ink formulation. In the typical formulation of ink, the role of SDS is to effectively disperse the filler nanoparticles in the solvent and thereby improve the colloidal stability of the ink. It will also help to decrease the coagulation of the ink. Ethanol acts as a vehicle for the uniform distribution of the filler and also controls the drying rate of the formulated ink. Because of the high volatile nature of ethanol, it facilitates the drying rate compared to aqueous-based screen-printable inks.[34,35] As is obvious, the purpose of the binder is to uniformly distribute the filler nanoparticles in the polymer matrix and to increase the adhesion of the fluorescent ink onto the substrate to which it is printed. Ideal proportional mixing of the filler, binder, and dispersants enables uniform printing and better adhesion of the printed pattern. In a stable ink suspension, the particle size and hence the mass of the dispersed filler phase should be as low as possible so that its kinetic energy is too small to overcome the electrostatic repulsion between charged layers of the dispersing vehicle phase. Hence, along with particle size, viscosity is a critical property of any ink for high-precision smooth printing. The rheological analysis of the formulated ink is given in Figure . For effective screen printing, the viscosity should be in the range of 1–10 Pa s.[36,37] From viscosity and shear stress plots, shown in Figure a, it is clear that with increased shear rate, the viscosity of the ink gets decreased. With the addition of SDS, colloidal stability of the formulated ink was maintained and the formulated ink showed a viscosity of 12 Pa s at a lower shear rate of 10 s–1 which further got decreased to about 9 Pa s at a higher shear rate. By increasing the shear rate, viscosity becomes substantially thinner and hence causes the ink to flow readily because of its pseudo-plastic behavior, which is clearly visible in the depicted plot. Another dynamic property of the ink is its viscoelastic nature, defined by the elastic/solid-like (storage modulus, G′) and viscous/liquid-like (loss modulus, G″) (see Figure b) behaviors. From the figure, it is clear that the value of G″ is higher than G′ in the given range of strain. This further confirmed the liquid-like behavior of the formulated ink, which is necessary to perform high aspect ratio screen printing.[38,39] Hence, the viscosity and dynamic viscoelastic properties of the ink are proved to be feasible for screen printing.

Figure 3

(a) Variation of viscosity with the shear rate. The inset given is the photographic image of the developed highly viscous ink and (b) dynamic viscoelastic properties of the developed double-layered fluorescent silica ink.

The photographic image of the viscous ink and printed patterns is shown in Figure . Fine screen printing can be achieved through a leveling process, and it depends upon certain factors such as rheology, surface tension, and flow time characteristics of the ink and also the mesh size and quality of both squeegee and screen.[40] During the screen printing process, the ink was forced to flow through the screen mesh openings, thereby producing high shear rates. As it passes through, the ink obtained its lowest viscosity, and once the screen is removed, the mechanical shear forces cease, and at the same moment, the ink is deposited onto the substrate. Thus, several patterns corresponding to the screen mesh could be printed on various flexible and rigid substrates. Figure a shows the NIIST-CSIR emblem screen-printed on a flexible Mylar film under visible light. It is worth noting that the printed pattern shows stable green emission under UV light (365 nm), as seen in Figure b. Figure c shows the bright fluorescence of the formulated ink under handheld UV light.

Figure 4

Photographs of (a) fluorescent ink printed on a flexible Mylar substrate, (b) under a handheld UV lamp, showing bright green emission, and (c) fluorescent ink under UV light.

Quality of printed films was examined through scanning electron micrographs, as depicted in Figure . The current ink formulation facilitates fast and natural drying of printed patterns typically less than 20 s because of the high volatile nature of the solvent chosen, ethanol, compared to aqueous-based screen-printable inks. The printed ink was found to be cured within seconds soon after the screen printing process, and this helps to produce a homogenized film without much porosity. This is clearly visible from scanning microscopy images. Figure a shows the low-resolution scanning electron microscopy (SEM) image of printed film, where the agglomerates of modified nanoparticles formed micron-sized spherical particles. From the high-magnification images of Figure b,c, it is evident that the filler particles were uniformly distributed, thus making the printing more or less uniform. However, along with uniform distribution, a fractional amount of porosity could be found in the microstructure, which might be as a result of high evaporation of organic solvents used in the ink preparation. Figure d shows the cross-sectional microstructure of the fluorescent ink printed on the Mylar film. The cross-sectional image clearly presents a clear morphological distinction between the printed film and the substrate, with a thickness of the printed layer around 15 μm. The thickness was found to be uniform throughout the layer, which further implies the homogeneity of the printed pattern.

Figure 5

(a) High-resolution surface image of the printed layer, (b,c) low-magnification surface images, and (d) cross-sectional image of the printed film showing thickness of the printed layer.

Figure illustrates the surface topographies of the screen-printed film. A clear microstructure of the printed fluorescent silica layer is clearly visible from the 3D AFM images. A typical 3D image of the printed film shows minor bumps and valleys, which can be minimized by using a hard flood, while holding the screen off the platen and also using a soft stroke during printing.[34,36] The unevenness of the printed film was measured from kurtosis topography, and a value of around 2.30 was obtained. If the surface has relatively high mountains and low valleys, the kurtosis value is less than 3, which is typically referred as platykurtoic.[41] A kurtosis value greater than 3 signifies the surface contains high peaks and low valleys, and the distribution curve is called as leptokurtoic. As seen, the present screen-printed fluorescent silica layer surface is platykurtoic. From AFM images, the average surface roughness (Ra) and the root-mean-square roughness (Rq) of the film were estimated and were found to be 87.3 and 107 nm, respectively. These values were in good agreement with the earlier reports on screen-printable inks.[34,36] Profile symmetry information was obtained from the skewness value of the film. Because bumps and valleys are predominant in the developed film, the expected skewness moment is negative. The present film gained a skewness of around −0.237, confirming the bumps and valleys nature of the film.[34] The obtained surface roughness and skewness are good enough for screen-printing applications.

Figure 6

(a) 2D and (b) 3D AFM images of the screen-printed film in the tapping mode.

Fluorescent studies of the as-synthesized double-layered silica nanoparticles and the printed surface with a suitable excitation wavelength spectrum were carried out. Both nanoparticles and the printed surface emit bright fluorescence with discrete peak emission. Absorption and emission properties of the synthesized fluorescent silica nanoparticles and the developed film are presented in Figure a,b, respectively. The absorption spectra of double-layered nanoparticles show bright and strong emission, confirming the successful doping of the dye within the silica matrix. The maximum emission wavelength of double-layered silica nanoparticles obtained is at 519 nm, which is more or less in concordance with earlier reports.[21] However, the emission intensity of the fluorescent printed pattern obtained was lower compared to that of the corresponding fluorescent silica nanoparticles. A similar trend was observed for excitation spectra of the printed film and fluorescent silica nanoparticles. Emission spectra of both powder and film were obtained upon excitation at a wavelength of 470 nm. As shown, the strongest fluorescence emission is observed for fluorescent double-layered nanoparticles at a wavelength of 530 nm upon excitation. The same excitation wavelength was used to excite the fluorescent silica-printed film on the Mylar substrate, where the emission was obtained at a wavelength of 525 nm. The emission wavelength and corresponding intensity of the printed film were slightly altered. The intensity discrepancy in both absorption and excitation spectra is mainly due to the additional components such as the binder and dispersant added for the formulation of rheostable ink. These additives are indispensable but may reduce the intensity counts of the film, showing less absorption and emission intensity compared to core–shell-structured fluorescent silica nanoparticles.[42] However, it is clear that the intensity of emission spectra of the printed film was not significantly affected by the ink components, which further confirms the fluorescence stability of the developed film to be similar to core–shell architecture. For practical applications, photostability of the printed film should be investigated through continuous irradiation. The normalized fluorescence data plotted against irradiation duration carried on for 180 min are shown in the inset of Figure b. Emission intensity was found to be decreased by 26% of its initial value at 60 min. After 180 min of continuous irradiation, the apparent drop was only 28%, as shown in the figure. These studies unveil the excellent photostability of the printed film over time, which is ideally suited for practical applications.

Figure 7

(a) Absorption spectra of synthesized fluorescent silica nanoparticles and developed film and (b) emission spectra of the film. Inset: Photostability of the printed film under continuous irradiation.

Conclusions

UV-readable fluorescent inks have potential applications in optical reading systems, intelligence information, and automatic identification systems. However, sustaining the fluorescence at the printed level requires a lot of optimization in synthetic chemistry and ink formulation. With the aim of developing a fast-curing fluorescent ink, we synthesized fluorescent silica nanoparticles through a modified Stöber method. Morphology and particle size of synthesized particles were analyzed through TEM micrographs. The rheostable viscous ink of fluorescent silica was formulated by choosing ethanol as the solvent system, whereas SDS and Butvar were used as the dispersant and the binder, respectively. The microstructure, surface roughness, and emission properties of the developed ink were measured. Even though the printed film got less intensity than corresponding fluorescent nanoparticles, the peak position of the film was not much affected. Hence, the developed ink provides promising applications in the fields of automatic identification, optical devices, and information encryption.