A quantum dot-spore nanocomposite pH sensor
Graphical abstract
Introduction
The measurement and control of pH are essential in chemical, biomedical, and environmental sciences and industrial applications. The development of new techniques for pH detection is attracting increasing interest [1], [2]. Potentiometric techniques employing a pH-sensitive membrane, such as glass membrane electrodes, are most commonly used for pH measurement. However, glass electrodes are subject to several issues, such as the influence of ionic strength, certain organic matter, and electromagnetic interference. An alternative involves developing new fluorescence pH sensors. Fluorescence sensors offer numerous advantages, such as small size, immunity to electromagnetic and radio frequency interference, and multiplexing capability.
In recent years, interest in the development of quantum dot (QD)-based pH sensors has increased because of the unique optical advantages of QDs in comparison with traditional organic dyes. The advantages of these sensors include the following: broad excitation spectra; tunable, narrow, and symmetric emission spectra; high resistance to photochemical degradation; and high photostability [3], [4], [5]. In this regard, a series of QDs has been tested as fluorescent platforms for pH sensing with different pH-responsive ranges, such as pH 7.1–8.5 for CdSe–ZnS core–shell QDs [6], pH 6.0–9.0 for thioglycolic acid (TGA)-capped CdTe QDs [7], pH 4.0–8.0 for mercaptoacetic acid-capped CdSe/ZnSe/ZnS QDs [8], and pH 6.0–8.0 for glutathione-coated CdSe/CdZnS QDs [9]. These studies indicated that QD-based pH sensors in aqueous solutions often lack the ability to sense responses over a wide pH range. In addition, colloidal QDs are not easily removed from solutions by conventional methods, such as filtration and centrifugation. Therefore, the use of colloidal QDs results in undesirable leaching, pollution, and inconvenient manipulation. To overcome these limitations, the development of hybrid materials based on the incorporation of QDs into solid matrices has drawn increasing interest. Previous studies demonstrated that the symbiotic effects of hybrid materials can yield novel properties and potential applications [10], [11], [12], [13], [14]. Among these techniques, polymer microparticles are commonly used as a QD carrier to form nanocomposites for the detection of biomolecules [15], [16], [17], [18], [19], [20], [21]. However, the use of QD-microparticle nanocomposites for pH sensing was rarely reported. In addition, polymer microspheres are traditionally prepared by physical and chemical methods [22], [23]. These traditional methods suffer from the requirement of hazardous chemical usage, which is harmful to the environment and human body.
Bacterial spores possess a rigid core and display remarkable resistance to malnutrition, heat, radiation, chemicals, and desiccation. These properties contribute to their applicability as vehicles for cell-based biosensors. In general, spore-based biosensors were developed based on a genetically engineered technique [24], [25]. However, the applications of engineered spore-based biosensors are still limited because of the long detection time, low sensitivity, and need for professional training, spore generation, and optimization for batch production, among others [26]. Recently, we successfully demonstrated that wild-type bacterial spores can be used for the antioxidant capacity [27] and phenol assay [28] on the basis of the existence of laccase (i.e., CotA protein) on the spore surfaces of Bacillus subtilis and Bacillus amyloliquefaciens.
Previously developed spore-based biosensors require living spores for analyte detection. In this study, we introduced a novel spore-based sensing method using the “dead” (inactivated) spores of B. subtilis as nanocarriers for pH detection. The procedure of pH sensing is shown in Scheme 1. First, B. subtilis spores were peeled off the loosely attached, balloon-like outer layers, which belonged to the exosporium [29], [30]. Second, the as-prepared spores were inactivated by using a high-pressure steam sterilizer for 30 min at 121 °C. The treated spores (TSs) could be considered as a bio-polymer microsphere for different applications. The proposed biological method for preparing spore-based microspheres presents several advantages, such as high-throughput, cost-effectivity, and ecological friendliness, over traditional physical and chemical methods. Third, TGA-modified CdTe/ZnS QDs were self-assembled onto TS surfaces to form QD@spore nanocomposites (i.e., QD microspheres) as detection devices. QD microspheres are expected to exhibit great potential for use as convenient, cheap, and effective pH-sensitive fluorescent probes for application in pH meters with a wide responsive range. Finally, the developed fluorescence microspheres were successfully used to determine the pH of water samples, yielding comparable results with those of commercial assay method.
Section snippets
Apparatus
A RF-5301PC fluorescence spectrophotometer (Shimadzu, Kyoto, Japan) equipped with 1 cm quartz cells was used for fluorescence measurements. A platform constant-temperature shaking incubator was purchased from Shanghai Jing Hong Laboratory Instrument Co. Ltd. (Shanghai, China). All solutions were prepared and diluted using ultrapure water (18.2 MΩ cm) produced by a Model Cascada IX laboratory ultrapure water system (Pall Co., Ltd., Washington, NY, USA). The pH of all buffer solutions was measured
Characterization of spore-based fluorescence microspheres
TEM images of primordial spores showed an irregularly shaped, loose-fitting, balloon-like layer, which is known as the exosporium, surrounding the outer layer of spores (Fig. 1A). The existence of exosporium could cause serious problems because of spore agglomeration as well as inhomogeneous distribution of QDs on different spores. In addition, QDs were mainly distributed within the exosporium when the QD solution was mixed with primordial spores. Hybrid exosporium/QDs were unstable and could
Conclusion
In summary, we have presented an efficient and facile protocol to generate uniform QD@spore nanocomposites by using bacterial spores as nanocarriers. This high-throughput approach is simple, inexpensive, and environmentally friendly. These unique nanocomposites could be used for small-volume sample analysis and recovered from samples to effectively overcome undesirable leaching and pollution caused by QDs. The developed fluorescence microspheres were successfully applied in pH detection with an
Acknowledgments
This work was supported by the National Science Foundation of China (no.81402723), Special Fund for Agro-scientific Research in the Public Interest (201303034-8), the Hi-Tech Research and Development Program of China (863 Program, no. 2012AA101304), the State Key Laboratory of Agricultural Microbiology, Huazhong Agricultural University (Grant no. AMLKF201405).
References (36)
- et al.
Introduction of quantum dots into PNIPAM microspheres by precipitation polymerization above LCST
Eur. Polym. J.
(2009) - et al.
Preparation of quantum dots encoded microspheres by electrospray for the detection of biomolecules
J. Colloid Interf. Sci.
(2011) - et al.
Fluorescent nanocrystal-encoded microbeads for multiplexed cancer imaging and diagnosis
Crit. Rev. Oncol. Hematol.
(2008) Functional polymer microspheres
Prog. Polym. Sci.
(2000)- et al.
What can spores do for us?
Trends Biotechnol.
(2003) - et al.
Whole-cell method for phenol detection based on the color reaction of phenol with 4-aminoantipyrine catalyzed by CotA laccase on endospore surfaces
Biosens. Bioelectron.
(2015) - et al.
Structure and assembly of the bacterial endospore coat
Methods
(2000) - et al.
Incorporating fluorescent quantum dots into water-soluble polymer
J. Lumin.
(2008) - et al.
Quantum dot incorporated Bacillus spore as nanosensor for viral infection
Biosens. Bioelectron.
(2015) - et al.
A microbent fiber optic pH sensor
Opt. Commun.
(2002)
A submersible autonomous sensor for spectrophotometric pH measurements of natural waters
Anal. Chem.
Self-assembly of CdSe−ZnS quantum dot bioconjugates using an engineered recombinant protein
J. Am. Chem. Soc.
Semiconductor nanocrystals as fluorescent biological labels
Science
Long-term multiple color imaging of live cells using quantum dot bioconjugates
Nat. Biotechnol.
pH-sensitive quantum dots
J. Phys. Chem. B
Green and orange CdTe quantum dots as effective pH-sensitive fluorescent probes for dual simultaneous and independent detection of viruses
J. Phys. Chem. B
pH-sensitive photoluminescence of CdSe/ZnSe/ZnS quantum dots in human ovarian cancer cells
J. Phys. C.hem. C
A quantum dot-based ratiometric pH sensor
Chem. Commun.
Cited by (17)
Cell-based fluorescent microsphere incorporated with carbon dots as a sensitive immunosensor for the rapid detection of Escherichia coli O157 in milk
2021, Biosensors and BioelectronicsCitation Excerpt :Many studies have shown that QDs and other nanomaterials can enter or attach to living cells and make the cells luminescent (Liu et al., 2015; Shang et al., 2014). The use of such luminescent cells (e.g., bacteria, spores, and yeast) as probes in bioanalyses has been demonstrated in several studies (Xiang et al., 2018; Zeng et al., 2016; Zhang et al., 2015, 2016a). Compared with traditional microspheres, those with biological cells as carriers are simpler to prepare and produce on a large scale.
Synthesis and characterization of multi stimuli-responsive block copolymer-silica hybrid nanocomposite with core-shell structure via RAFT polymerization
2020, Composites Science and TechnologyCitation Excerpt :These nanocomposites have several applications in various fields such as homogeneous and heterogeneous catalysis, optical devices, drug delivery, tissue engineering, and sensors [19]. Polymeric nanocomposites sensors are among the most famous sensors and are of numerous types namely; gas sensors [20], touch sensors [21], temperature sensors [22], photo [23] detectors, and last but not least, pH sensors [24]. These properties originate from the multi stimuli-responsive polymers that are used for the synthesis of these nanocomposites [25].
Functionalized nanomaterials for chemical sensor applications
2020, Handbook of Functionalized Nanomaterials for Industrial ApplicationsFluorescent pH nanosensors: Design strategies and applications
2019, Journal of Photochemistry and Photobiology C: Photochemistry ReviewsGraphene as biomedical sensing element: State of art review and potential engineering applications
2018, Composites Part B: EngineeringCitation Excerpt :The broad term sensors is an electronic module, component, subsystem or an element based on the certain detection mechanism to measure the changes in the environmental activities, [1–5].
Protein self-assembly via Zr<sup>4+</sup> ions on spore-based microspheres for immunoassays
2018, Sensors and Actuators, B: ChemicalCitation Excerpt :However, cell-based microspheres are still limited due to their insufficient mechanical strength, which causes the microspheres to collapse and break. We recently introduced an innovative biological approach using Bacillus spores for preparing highly monodisperse microspheres (MMs) that feature different functionalities (i.e., carboxylic, amino, and hydroxyl groups) and demonstrated the remarkable potential of this novel class of bio-derived microspheres as advanced functional materials for tailored applications [21–23]. Moreover, these MMs have ideal physical properties, such as excellent mechanical strength (due to the presence of a protective shell), uniform size distribution, a higher density than water, and a dimension of approximately 1 μm.