Colloidal silica beads modified with quantum dots and zinc (II

Microchim Acta (2013) 180:85–91 DOI 10.1007/s00604-012-0914-2

ORIGINAL PAPER

Colloidal silica beads modified with quantum dots and zinc (II) tetraphenylporphyrin for colorimetric sensing of ammonia Hua Xu & Maochun Zhang & Haibo Ding & Zhuoying Xie

Received: 24 July 2012 / Accepted: 5 November 2012 / Published online: 14 November 2012 # Springer-Verlag Wien 2012

Abstract Colloidal crystal beads (CCBs) were fabricated by assembling monodisperse silica nanoparticles via a microfluidic device. The pore size of the CCBs was tuned by using different nanoparticles. The CCBs were then coated with cadmium telluride quantum dots and zinc(II) mesotetraphenylporphyrin for the purpose of optical sensing. Ammonia causes the color of the sensor to change from green to red. The method has a dynamic range of 0–2500 ppm, good reversibility, and is not sensitive to humidity. The limit of detection is 7 ppm. The sensor has the advantage of a porous microcarrier structure and that pore sizes can be well controlled and thus can fulfill various demands in gas detection. Keywords Colloidal crystal beads . Colorimetric ammonia sensing . Fluorescence . Porphyrin . Quantum dots

Introduction Qualitative and quantitative analyses of ammonia in the environment are great important in environmental monitoring, public security and industrial monitoring [1]. Colorimetric

Electronic supplementary material The online version of this article (doi:10.1007/s00604-012-0914-2) contains supplementary material, which is available to authorized users. H. Xu (*) : M. Zhang : H. Ding : Z. Xie State Key Laboratory of Bioelectronics, School of Biological Science and Medical Engineering, Southeast University, Nanjing 210096, People’s Republic of China e-mail: huaxu@seu.edu.cn H. Xu Suzhou Key Laboratory of Environment and Biosafety, Research Institute of Southeast University in Suzhou, Suzhou 215123, People’s Republic of China

detections of ammonia have attracted much attention due to their special advantages, including rapidly readable responses, and a feasible and simple detection approach. PH indicators are widely used chemochromic reagents for colorimetric sensing of ammonia, which based on the lewis acid–base reaction and induced the color change due to hydrogen release [2–10]. In the past decade, metalloporphyrin was considered as one of the most promising materials for gas sensing because they can react with gases at room temperature and their reactivity can be finely tuned by modifying the molecular structure or coordinated metal [11, 12]. Moreover, their chromophoric property is related with the interaction with gas molecules, which can be derived for the construction of colorimetric gas sensing. So far, a number of colorimetric sensors based on metalloporphyrin have been reported [12–17]. As a practice sensing material, the metalloporphyrin should be deposited onto a substrate and the substrate microstructure is a critical factor affecting the performance of the sensor [18–20]. Porous-structured substrate is preferred to increase the surface to volume ratio and so as to increase the sites for gas adsorption. A large amount of porous materials such as nanofiber [21–23], carbon nanotube [24–26], and porous ormosils [27–30] have been applied for fabrication of high performance metalloporphyrins film sensor. When these porous films were integrated with metalloporphyrins for colorimetric sensing, the observation angle of the color is narrow and the best position is located at the normal direction of the film, which prevent the observation of the colorimetric change in some special circumstances, such as gas detection in a narrow crack. This problem can be solved by making the spherical porous support. Furthermore, it has been well illustrated that the smaller pores usually mean larger specific surface area and, thus, resulting in higher sensitivity. On the other hand, the smaller pores usually mean slower gas diffusion, which determine the reaction and recovery speed in

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colorimetric sensor [31]. Therefore, it is important that the fabrication of the porous material with controllable pore sizes are satisfied the different requirements of gas analysis and detection. Colloidal crystal bead (CCBs), which is spherical porous material composed of monodisperse nanoparticles, have been fabricated and used as mobile carriers to immobilize probe molecules for rapid detecting target biomolecules [32–35]. In this article, the CCBs were fabricated by a microfluidic device and their pore sizes were tuned by using different nanoparticles. Cadmium telluride (CdTe) quantum dots (QDs) and zinc(II) meso-tetraphenylporphyrin (ZnTPP) were coated and functionalized the CCBs for colorimetric ammonia sensing. Distinct color change of the sensor was observed to ammonia with concentration from 100 to 2500 ppm. The reversibility, the humidity effect and the reusability of the QD-ZnTPP CCBs sensor were further investigated.

Experimental Reagents and instruments Polytetrafluoroethylene (PTFE) pipes with inner diameter of 500 μm were purchased from Upchurch Scientific, Oak Harbor, WA, USA (http://webstore.idex-hs.com). Polydimethylsiloxane (KF-96 10 cSt) and Poly (1,1,1-trifluoropropylmethylsiloxane) (x-22-821) were the product of Shin-Etsu Chemical, Japan (http://www.shinetsu.co.jp/e/index.shtml). Fluoroalkylsilane (FAS, AY43-158E) was gift from Hongbin Chemicals LTD., China. Cadmium telluride (CdTe) quantum dots (QDs) [36] and zinc(II) meso-tetraphenylporphyrin (ZnTPP) [37, 38] were prepared according to literature procedures. Monodisperse silica nanoparticles were synthesized according to a modified Stöber method [39]. 50 % NH3 (other is 50 %N2) was purchased from Nanjing Special Gas Co. Ltd (China) and diluted to different concentrations with N2 by mass flow controller. Different relative humidity was generated by different saturated salt solution [40]. SEM images were obtained using a scanning electron microscope (SEM, Hitachi S-3000 N). The CCBs were observed using an optical microscope (Olympus BX-51) and images were recorded using a charge coupled device (CCD) (MediaCybernetics Evolution MP 5.0 RTV). Fluorescence images were captured using a fluorescence microscope (Olympus IX71). Fluorescence spectra of the CCBs were recorded using an optical microscope equipped with a fibre optic spectrometer (Ocean Optics, USB2000-FLG). Fabrication of the CCBs with controllable pores size A small hole was drilled by excimer pulse laser on the surface of PTFE pipe and then a fluorinated dispenser needle was

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inserted into the hole to form a drop breaking installation. All the junction points were sealed by waterproof glue. An aqueous suspension containing monodisperse silica nanoparticles with a concentration of 10 % (w/v) and the silicone oil KF-96 containing x-22-821 (30 %v/v) and Span-80 were simultaneously injected into the PTFE tube. The aqueous suspension was broken into droplets by the oil flows and the droplets were collected into a container filled with the silicon oil. The water in the droplets was evaporated at 70 °C, and solid beads were derived after 12 h. After solidification, the beads were thoroughly washed with hexane to remove the silicone oil. Silica nanoparticles with different size were used to control the pore size of the CCBs. Fabrication of QD-ZnTPP modified CCBs The CCBs were calcined at 800 °C for 3 h to improve the mechanical stability and then immersed in an acetone solution of 3-aminopropyltriethoxysilane. After being washed with acetone and dried, the CCBs were immersed in toluene solution of CdTe QDs to immobilize on CCBs by physical adsorption. After complete dryness of the CdTe QD modified CCBs, the chloroform solution of ZnTPP (1 mg.mL−1) were deposition on the CCBs by solvent-casting method. Finally, the QD-ZnTPP modified CCBs were washed with chloroform to remove excess porphyrin and dried in the dark at room temperature. Colorimetric sensing properties to ammonia The QD-ZnTPP modified CCB was placed in a 25 mL test chamber with an inlet, an outlet, and a transparent glass cover. First, the original fluorescence intensity (I0) and the color pattern of the sensor before ammonia exposure was recorded under a nitrogen atmosphere. After exposure to the ammonia gas, the fluorescence intensity (I) and color pattern of the sensor were recorded at regular intervals until the variation reached equilibrium. After each measurement, the quenched sensor was exposed to nitrogen to recover.

Results and discussion Fabrication of the CCBs with controllable pores size A fluorinated dispenser needle was inserted into the PTFE pipe to form a simple drop generating installation, as shown in Fig. 1. When the aqueous suspension containing silica nanoparticles and the silicon oil were simultaneously injected into the PTFE tube, the aqueous suspension was broken into droplets by the oil flows at the needle tip. The oil took the suspension droplets into the collection container which was filled with the silicon oil. The as-prepared droplets act as

Beads modified with QD and porphyrin for sensing of ammonia

Fig. 1 Illustration of a simple microfluidic device for the fabrication of CCBs

template to generate CCBs by heating-induced the aggregation of the nanoparticles. By this method,three CCBs which were assembled by 154 nm, 270 nm, and 428 nm silica nanoparticle respectively were fabricated and their SEM image shown in Fig. 2(a-d). It can be observed that the beads are spherically shaped in Fig. 2 (a) and the nanoparticles in the beads all formed hexagonally close-packed ordered structure in enlarged SEM image. Fig. 2 SEM images of 200 um colloidal crystal beads. (a) lowmagnification image of beads composed of 154 nm silica nanoparticle. Highmagnification images of beads composed of 154 nm (b), 270 nm (c) and 428 nm (d) silica nanoparticles with the pores size of about 20 nm, 40 nm and 60 nm, respectively. Fluorescence microscopic images of CdTe QDs modified CCBs (e) and CdTe QDsZnTPP modified CCBs (f)

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There is a pore among every three nanoparticle touched each other and the morphology and the size of the pores in every CCB are uniform. As the pores were formed by the hexagonally close packing of the nanoparticles, it can be deduced the pores size is proportion to the size of the nanoparticles. The larger the nanoparticles are, the larger will be the pores size. SEM image of the bead in Fig. 2(b-d) exhibited with the increase of silica nanoparticle from 154 nm to 428 nm, the pores also increase from about 20 nm to 60 nm. This result indicated the pores size of the CCBs can easily be tuned by simple using the different nanoparticles. The porous structure of the CCBs provided a large surface-to-volume ratio and a large number of channels for gas diffusion. Therefore, a high sensitivity would be expected using the encoded CCBs as a carrier for gas analysis and detection. Preparation of QD-ZnTPP modified CCBs To improve colorimetric limitation and quantitative ammonia determination, a single of layer of CdTe QDs was coated

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C NH 3 /ppm

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Fig. 3 Apparent colors of the QDs-ZnTPP modified CCBs sensor to different concentrations of ammonia in equilibrium forming “ammonia ruler” for the colorimetric determination

on the CCBs as background [41]. Then the ZnTPP was coated on QD-modified CCBs surface as ammonia-sensing layer. The CCBs were first treated with trimethoxy(octadecyl)silane to improve the surface hydrophobicity to achieve a uniform absorption, and then the hydrophobic QD and porphyrin was coated onto the CCBs respectively via physical adsorption. SEM image of CdTe QD-ZnTPP modified CCBs showed there is no damage to the original porous structure of the CCBs after coating them with the porphyrin, and that the porphyrin molecules on the bead surface were uniform (Fig. S1). Fluorescence microscopic images of CdTe QD-modified CCBs and CdTe QD-ZnTPP modified CCBs are shown distinct green and red in Fig. 2e and f respectively, which is assigned to the fluorescence color of CdTe QD and ZnTPP. The uniform fluorescence color of CdTe QD-modified CCBs and CdTe QD-ZnTPP modified CCBs also indicated that the CdTe and ZnTPP fluorophores were homogeneously distribution on CCBs, which is important to colorimetric determination to generation of a homogeneous color. As the CCBs microcarriers is spherical and the distribution of the CdTe and ZnTPP fluorophores on CCBs were homogeneous, the fluorescence color of CdTe QD-ZnTPP modified CCBs observed from different angle all showed distinct red (Fig. S2). The result

indicated the fluorescence color of the CCBs sensor can be observed with a wide observation angle. Colorimetric Gas sensors for the detection of ammonia Figure 3 shows the response of the CdTe QD-ZnTPP modified CCBs towards various concentrations of ammonia at equilibrium. Under different ammonia concentration, the sensor displayed distinguishable colors change from red to green with a resolution up to 100 ppm, which could easily be identified with the bared eyes or a CCD camera. The change of color originates from luminescence quenching of ammonia-sensing probe (ZnTPP). In the absence of ammonia, the ZnTPP layer emits a red color and the red luminescence can be quenched when increasing the ammonia concentration. Furthermore, the CdTe QDs layer was stable and insensitive towards ammonia and served as a green background. Therefore, the color of the sensor represented red in the absence of ammonia. When gradually increasing the ammonia concentration, the color of the sensor gradually changed from red to green due to luminescence quenching of the ZnTPP layer. The apparent colors of the sensor in different concentrations of ammonia can acts as an optical “ammonia ruler” for the colorimetric determinations (Fig. 3). Ammonia concentration could easily be directly readout using the 80

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Fig. 4 Fluorescence intensity change of the QDs-ZnTPP modified CCBs sensor in different concentrations of ammonia (lem 0650 nm). I0 and I represented the fluorescence intensity of the sensor before and after exposure to ammonia respectively

Fig. 5 Reversible fluorescence intensity response of colorimetric ammonia sensor when alternately exposed to different concentrations of ammonia and nitrogen

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consistent with the color change of the sensor to ammonia. As shown in Fig. 4, when the sensor was exposed to ammonia in different concentrations, the fluorescence intensity ratio (I0/I) of the ZnTPP layer all first displayed an obvious increase. This result is coincident with the color of the sensor to ammonia changed obviously. With the increase of the ammonia concentration, the fluorescence intensity ratio (I0/I) also increased. This result indicated more effective luminescence quenching occur in the high concentrations of ammonia, which is consistent with more apparent color change of the sensor in high concentrations ammonia (Fig. 3). Furthermore, the curve (Fig. 4) shows a good degree of linearity in low concentration range. The LOD of ammonia was defined as the concentration at which the signal is equal to the blank signal plus 3 s [12]. Using this method the LOD is calculated to be about 7 ppm. Reversibility and humidity insensitivity

“ammonia ruler”, which made the ammonia determination rapid, feasible and simple. Fluorescence intensity change of the QD-ZnTPP modified CCBs sensor in different concentrations of ammonia also indicated effective luminescence quenching, which is

When the used sensor was exposed to nitrogen gas, the color and the fluorescence intensity of the sensor all presented good recovery, as shown in Fig. 5. After continuous exposure to different concentrations of ammonia and nitrogen

Table 1 Figures of merits of comparable methods for sensing ammonia Chemochromic reagents/Matrix

Methods (NH3 form)

Analytical ranges and LODs (ppm)

Comments

Reference

Bromophenol Blue/silicone film

Col. (dis.) Fl. (dis.)

PH effect, limited operational lifetime, affected by cationic irreversibly, applied for “single shot” tests

[7]

Rhodamines/PVC membranes Aminofluorescein/Ormosils film

Fl. (dis.)

0.01–17 0.01 0.1–10 0.1 1–20 1

long operational lifetime, good photostability affected by PH

[9]

Phenol red, uponversion nanoparticle/PS matrix Bromophenol blue/plastic film

Fl.(dis.)

applied for sensing ammonia in complex matrixes

[10]

Col. (g)

PH effect, temperature influence

[2]

Bromophenol blue/PMMA matrix

Col. (g)

[3]

Bromophenol blue/PVB + TBP film

Col. (g)

Bromothymol blue/Amberlite polymer beads ZnTPP/silicone films

Ref. (g)

PH effect, RH influence, waveguide configuration effort low-cost, poor stability, optical fiber waveguide Instrumental effort PH effect, dimethylamine influence, instrumental effort reversible and reproducible, triethylamine influence

MnTPP/nitrocellulose membrane

Abs. (g)

low LOD, narrow detection range

[14]

ZnTPP/DEP-PVC membranes

Abs. (g)

good stability in the dark, not specific for ammonia

[15]

ZnTPP, QD/colloidal silica beads

Fl./Col. (g)

spherical structure with controllable pores size, readable colorimetric detection, applicable to quickly detection of ammonia leaked from some narrow places

This method

Fl. (g)

40–800 16 3–1100 3 0.25–20 0.25 10–1000 5 33–1344 33 0–50 0.7 0–100 0.63 14–2950 4 0–2500 7

[8]

[4] [5] [13]

Co1. colorimetric; F1. fluorimetric; Abs. absorption; Ref. reflectance; (g) ammonia in gaseous form, (dis) ammonia dissolved in aqueous solution

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atmosphere, 95 % of the fluorescence intensity can be recovered and the color of the sensor is completely reverted back to its original state. The very good reversibility of the sensor is due to the excellent reversible property of ZnTPP, which recognized ammonia by reversible metal coordination interaction between zinc ion and ammonia. In this way, the sensor could be reused many times, which is important in developing low-cost colorimetric sensing systems. One of the most serious weaknesses in current sensor technology is sensitivity to changes in humidity. Because the porphyrin and QD in our sensor is hydrophobic, waterinsoluble dye and the CCBs microcarriers on which the porphyrin and QD was coated are also hydrophobic, the sensor is essentially impervious to changes in relative humidity (RH). As shown in Fig. 6, the sensor is essentially unresponsive to water vapor at various RH conditions ranging from 11 % to 97 %. Similarly, the response to ammonia is not affected by the presence of RH over this range. The water vapor insensitivity of our sensor provides a substantial advantage in the analysis of real-world samples. Comparable methods for sensing ammonia Up to now, a number of dye-based sensing films have been developed for detection of ammonia in gaseous form or dissolved in aqueous solution, most of which are listed in Table 1. Compared to other ammonia sensor, our CCB sensor have their special advantages, including their spherical structure with the size of micron, controllable pores size, readable colorimetric detection by bared eyes, wide observation angle and is especially applicable to quickly detection of ammonia leaked from some narrow places.

Conclusion In conclusion, CCBs were fabricated by assembling monodisperse nanoparticles through a microfluidic device. The pores size and the specific surface area of the CCBs can easily be tuned by using different nanoparticles. The CCBs were coated CdTe QDs and ZnTPP and used for a readable colorimetric ammonia sensing. Ammonia causes the color of the CCBs sensor to change from green to red with a wide dynamic range of 0–2500 ppm. Here, we note that this new type of ammonia sensor has attractive features including its porous spherical structure, simple preparation, good reversibility, insensitivity to humidity, controllable pores size, and its size of micron, readable colorimetric detection by bared eyes and wide observation angle, make it especially applicable to quickly detection of ammonia leaked from some narrow places. In particular, such a detection technique foresees a high potential for the detection of other gases by changing the colorimetric reagent.

H. Xu et al. Acknowledgments This work was supported by NSFC (Grant No. 21103020, 50925309) and the Suzhou Science and Technology Department (Grant No. SYG201209, SH201110).

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