Synthesis, Characterization and Optical Properties of CdSe@CdS and CdSe@ZnS Core/Shell nanoparticles for Bioimaging Applicaton

 D K Gupta^a,b*, N.D. Jasuja^c Gaurav Sharma^d & K B Sharma^b

^aCentre for Converging Technologies, University of Rajasthan, Jaipur 302004, India

^bSemi-conductors and Polymer Science Laboratory, Department of Physics, University of Rajasthan, Jaipur 302004, India.

^cDepartment of Biotechnology and Allied Sciences, Suresh Gyan Vihar University, Jaipur.

^dSchool of Applied Sciences, Suresh Gyan Vihar University Jaipur

*Email: deepak.nanoconverge@gmail.com

Abstract:  A study was conducted to investigate Synthesis, Characterization and Optical Properties of Semiconductor@Semiconductor (CdSe@CdS and CdSe@ZnS) Core/Shell nanoparticles. It was found that concentric spherical core/shell nanoparticles were the most common where a simple spherical core particle was completely coated by a shell of a different material. Core–shell nanoparticles (CSNs) are a class of nanostructured materials that have recently received increased attention owing to their interesting optical properties and broad range of applications in optoelectronic and bioimaging field. By rationally tuning the cores as well as the shells of such materials, a range of core–shell nanoparticles can be produced with tailorable properties that can play important roles in various applications.

Keywords: A. core/shell nanoparticles, B. Synthesis Approaches, C. Characterization Techniques, D. Optical properties.

1. Introduction

The quantum dot is zero dimensional semiconducting nanocrystal whose radii is smaller than the bulk Bohr exciton radius. The electronic and optical properties of these nanomaterials show a significant change from their corresponding bulk properties, which are called quantum size effects. These QDs make a class of materials intermediate between molecular and bulk forms of matter. Quantum confinement of both the electron and hole in all three dimensions leads to an increase in the effective band gap of the material with decreasing crystallite size. The optoelectronic properties of semiconductor nanocrystals or quantum dots are dimension dependent in the nanometer range such as size dependent band gap, which leads to control and tune material’s properties. Consequently, both the optical absorption and emission of quantum dots shift to the blue (higher energies) as the size of the dots gets smaller due to more pronounced quantum properties. Due to broad absorption spectra, solar devices using quantum dots have more efficiency than others. Quantum dots have good transport and optical properties due to very sharp density of states, hence can be used for LED and LASER devices. Quantum dots can be used for different applications due to their size and shape dependent properties such as display devices, photovoltaics, lasers and biomedical imaging^1-9. CdSe quantum dots (QDs) have attracted interest in the fields of optoelectronics and biomedical imaging due to their wide absorption cross sections and narrow emission bands ^10-11.

Semiconductor nanocrystals (SNCs) have drawn considerable attention due to the formation of core/shell nanoparticles¹². The QDs require an emission that is stable against photo and chemical degradations for the use as light emitters. These characteristics can be achieved by coating the QD surface with inorganic materials which have a broader band gap that encompasses the band gap of the core QD^13-14.

      Overcoating of nanocrystals with higher band gap inorganic materials can be used to improve the photoluminescence quantum yields by passivating surface nonradiative recombination sites. Particles passivated with inorganic shell structures are more robust than organically passivated dots and have greater efficiency to process the conditions required for integration into solid state structures^15-23. Some examples of core-shell nanoparticle structures reported include CdS on CdSe and CdSe on CdS¹⁵, ZnS grown on CdS¹⁶, ZnS on CdSe and the inverse structure¹⁷, CdS/HgS/ CdS quantum dot quantum wells¹⁸, ZnSe over coated CdSe¹⁹, and SiO2 on Si^20-21. In addition to having higher efficiencies, CdS and ZnS over coated particles are more robust than organically passivated dots and potentially more useful for optoelectronic device structures. Electroluminescent devices (LED’s) incorporating quantum dots (CdSe@CdS or CdSe@ZnS) into heterostructure organic/semiconductor nanocrystallite light-emitting devices may show greater stability^23-24.

The core-shell nanocomposites and nanostructure may be with different sizes and different shapes of core and shell thickness with different surface morphology. They may be spherical, centric, eccentric, star-like, or tubular in shape. Depending on the size and shape, their properties tune from material to another. Individual core-shell nanoparticles have various applications in diverse fields of medical biotechnology, like molecular bioimaging, drug delivery, cancer therapy, and so forth. Whenever the surface of the nanoparticles is modified by functional groups or molecules or coated with a thin layer of other materials (with different constituents), they show enhanced properties compared to the nonfunctionalized uncoated particles²⁵.

2. Scope of This Review

Reviews play an important role in keeping interested parties up to date on the current state of the research in any academic field. This review aims to focus on the development of the three different aspects of core/shell nanoparticles. These are (i) synthesis approaches, (ii) characterization techniques, and (iii) Importance of core/shell nanoparticles (iv) Classfication (v) Application due to optical properties. Since core/shell nanoparticle synthesis and its applications are relevant emerging research areas in nanotechnology, over the last 2 decades different research groups have synthesized and studied the properties and applications of these different core/ shell nanoparticles. To date some researchers have also published some reviews as well as book chapters²⁶ in this field mainly highlighting some specific material properties such as Au/polymer²⁷, polymer/silica²⁸, silica/biomolecules²⁹, organic/inorganic³⁰, organic/ organic,74 organic coated core/shell³¹, magnetic³², semiconductor³³, etc. or applications of some specific core/shell particles.Finally, keeping in mind the importance of advanced materials in today’s world, it is hoped that there is still a strong demand for an extensive review with updated literature on core/shell nanoparticles³⁴.

3. Synthetic and characterization techniques of core–shell nanoparticles

As mentioned earlier, the recent upsurge in the field of CSNs can be attributed to their diverse applications because of their unique structural, physical and chemical properties³⁵. Although the review primarily focuses on the optical properties of CSNs, the synthetic strategies, used to make assemblies of CSNs as well as different characterization techniques are briefly described in the following sections.

3.1.   Synthesis of core–shell nanoparticles

As far as synthesis is concerned, the techniques that have been used to synthesize nanomaterials can generally be applied to prepare the core and/or shell components of CSNs. Although ‘‘top-down’’ approaches are possible, in which external controls like microfabrication techniques, mechanical stress, etc. are utilized to break down the bulk materials to the desired nanomaterials with different shapes and sizes, ‘‘bottom-up’’ techniques are mostly preferred. In the latter case, the materials are synthesized from molecular or atomic building blocks, relying on the inherent chemical properties of the individual constituents and their mutual interactions to allow control over the shape and size of the nanomaterials. Using bottom-up synthetic methods, core and shell(s) of CSNs can be synthesized either in a stepwise or one-pot fashion. Both of these approaches have been used for the synthesis of CSNs. However, other new and modified procedures are continuously being developed and reported in the literature^36-37.

• Characterization of core/shell nanoparticles

The characterization of core/shell nanoparticles is critical because of the presence of shell material on the core surface; hence, a suitable characterization technique is always required for both the core and shell. Most characterization techniques used are the same as those used for single particles, but one technique may not be sufficient. The most significant characterization techniques used for core/shell nanoparticles are those for the measurement of size, shell thickness, elemental and surface analysis, optical properties, and thermal stability among others. Therefore, the usual characterization techniques such as dynamic light scattering (DLS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and UV-Vis spectroscopy are the ones most often used. Depending on the characterization techniques and different instruments, analysis can be classified as described in the following sections^38-40.

4. Importance of Core/Shell Nanoparticles

Core/shell nanoparticles are gradually attracting more and more attention, since these nanoparticles have emerged at the frontier between materials chemistry and many other fields, such as electronics, biomedical, pharmaceutical, optics, and catalysis. Core/shell nanoparticles are highly functional materials with modified properties. Sometimes properties arising from either core or shell materials can be quite different. The properties can be modified by changing either the constituting materials or the core to shell ratio⁴¹. Because of the shell material coating, the properties of the core particle such as reactivity decrease or thermal stability can be modified, so that the overall particle stability and dispersibility of the core particle increases. Ultimately, particles show distinctive properties of the different materials employed together. This is especially true of the inherent ability to manipulate the surface functions to meet the diverse application requirements^42-43. The purpose of the coating on the core particle are many fold, such as surface modification, the ability to increase the functionality, stability, and dispersibility, controlled release of the core, reduction in consumption of precious materials, and so on.

Current applications of different core/shell nanoparticles are summarized in a review article by Karele et al.⁴⁴ The individual reports from different researchers also demonstrates the fact that core/shell nanoparticles are widely used in different applications such as biomedical⁴⁵ and pharmaceutical applications⁴⁶, catalysis⁴⁷, electronics⁴⁸, enhancing photoluminescence⁴⁹, creating photonic crystals⁵⁰, etc. In particular in the biomedical field, the majority of these particles are used for bioimaging⁵¹, controlled drug release⁵², targeted drug delivery⁵³, cell labeling and tissue engineering applications^54-55. In addition to the improved material properties, core/shell materials are also important from an economic point of view. A precious material can be coated over an inexpensive material to reduce the consumption of the precious material compared with making the same sized pure material. Core/shell nanoparticles are also used as a template for the preparation of hollow particles after removing the core either by dissolution or calcination. Nano- and microsized hollow particles are used for different purposes such as microvessels, catalytic supports⁵⁶, adsorbents⁵⁷, lightweight structural materials⁵⁸, and thermal and electric insulators⁵⁹.

5. Classification of core/shell nanoparticles

There are large varieties of core/shell nanoparticles available so far with a wide range of applications. As a result, the classification of all the available core/shell nanoparticles, which depends on their industrial applications or is based on some other property, is a challanging task. In this review, we attempt to classify the core/shell nanoparticles depending on their material properties. In a broad sense, clearly the core or shell materials in a core/shell particle are either made of inorganic or organic materials. Depending on their material properties, the core/shell nanoparticles can be classified into four main different groups: (i) inorganic/inorganic; (ii) inorganic/organic; (iii) organic/inorganic; (iv) Organic/organic.

5.1. Semiconductor Core/Shell Nanoparticles.

Semiconductor nanoparticles are also known as quantum dots (QDs). One definition, describes a quantum dot as a semiconductor where excitons (electron and hole) are confined within all three spatial dimensions. The recommended band gap for semiconductor particles is normally greater than that for conductor materials but less than 4 eV, which are those normally found in insulating materials. In the early years of this research, pure group IVA (Si, Ge) elements were used as the semiconductor material, whereas later, compounds of different element groups, such as IIIAVA, IIBVIA, and IBVIIA, also became popular as semiconductor materials. The lattice spacing of these different materials is almost the same with the only difference the fact that the bonds have a partially ionic character. This increases to a certain extent moving from left to right in the periodic table. Because of overlapping of the valence and conductance bands of two elements, the band gap also changes; either increasing or decreasing with respect to the pure semiconductor. The band gaps of some common semiconductor materials are given in Table 2. For the pure elements within a particular group, it can be seen that as we move from top to bottom the band gap decreases. Similarly, for the compounds of a particular element if other elements are changed in a group, the band gap gradually decreases while moving from top to bottom. With respect to the core/shell nanoparticles discussed in this section, either both the core and shell are made of semiconductor materials or one is a semiconductor and the other is a nonsemiconductor material. So, depending on the material properties used the semiconductor core/shell nanoparticles can be classified as follows: (i) semiconductor/nonsemiconductor core/shell nanoparticles or (ii) semiconductor/semiconductor core/shell nanoparticles. Both types of particles are used for medical or bioimaging purposes⁶⁰, enhancment of optical properties⁶¹, light-emitting devices⁶², nonlinear optics⁶³, biological labeling⁶⁴, improving the efficiency of either solar cells⁶⁵ or the storage capacity of electronics devices⁶⁶, modern electronics field applications⁶⁷, catalysis⁶⁸ etc.

5.2. Semiconductor/Semiconductor Core/Shell Nanoparticles.

Over the past 2 decades, instead of using either a single semiconductor material or semiconductor/nonsemiconductor core/shell material, researchers have been using semiconductor/semiconductor core/shell materials to improve the efficiency and decrease the response time. Of particular interest is where both the core and shell are made of a semiconductor material or a semiconductor alloy⁶⁹.These types of particles are used as either binary materials with core and shell or tertiary materials, that is, a core with a double shell coating. In most common core/shell quantum dots, the core and shell are mainly made of alloy materials. Apart from reviewing the relevant research papers, different aspects of the semiconductor/semiconductor core/shell materials have also been extensively reviewed by Reiss et al.⁷⁰ The main advantages of such particles are the fact that because they have an external coating of another semiconductor material, this increases optical activity and photooxidation stability. Depending on their relative energy levels of the valence and conductance bands and the band gap of the core and shell materials, these semiconductor/semiconductor core/shell nanoparticles can also be classified into three different groups.

5.3. Shell Materials with Higher Band Gaps.

In this category, the energy band gap of the shell material is wider than the band gap of the core material. The electrons and holes are confined within the core area because both the conduction and the valence band edges of the core are located within the energy gap of the shell⁷¹. The valence band of the shell is also at a lower energy than that of core. This arrangement of energy levels is essential in order to confine electrons and holes within the core material. The shell is used to passivate the surface of the core with the goal of improving its overall optical properties. Another role of the shell is to separate the more optically active core surface from its surrounding environment. The wider band gap shell material increases the stability against photobleaching of the semiconductor core. However, the increasing thickness of the shell layer reduces the material surface activity of the core surface; as a result, quantum yield also reduces. With increasing shell layer thickness, a small red shift occurs for the UVvis absorption spectra and the PL wavelength compared with that of uncoated core. These types of semiconductor particles especially those made from CdSe/CdS⁷²,CdSe/ZnS⁷³ or CdTe/CdS⁷⁴ materials, have been extensively studied by different research groups. Liu and Yu⁷⁵ studied the absorption and emission spectra of CdTe and CdScoated CdTe nanoparticles and results showed that the absorption and emission peak positions are shifted to a higher wavelength with increasing reaction time . This is a result of the coating of CdS, the higher band gap material, on the lower band gap material, CdTe. In this case, the QY initially increases, but again with increasing reaction time, QY decreases because of the increase in shell thickness.

6. Application due to Optical properties

Overcoating of nanocrystals with higher band gap inorganic materials can be used to improve the photoluminescence quantum yields by passivating surface nonradiative recombination sites These core/shell nanoparticles are therefore very promising for many applications where optical properties need to remain stable, such as lasing or biolabeling⁷⁶.

Different types of molecular imaging techniques, such as optical imaging (OI), magnetic resonance imaging (MRI), ultrasound imaging, positron emission tomography, and others are used for the imaging of both in vivo and in vitro biological specimens. However, optical and magnetic resonance imaging techniques are the most acceptable because they utilize the inherent luminescent and magnetic properties of the nanoparticles. The two principal types of nanoparticles that have been used for imaging in vivo systems are luminescent nanoprobes for optical imaging and magnetic nanoparticles for magnetic resonance imaging. In some specific cases, dual-mode nanoparticles are used for simultaneous optical and magnetic resonance imaging^77-80.

Acknowledgments

Professor N.S. Saxena gratefully acknowledges UGC, New Delhi (India) for providing financial support in the form of Emeritus fellowship for the preparation of core/shell nanoparticles. The authors also thank DST-FIST II for UV-Vis spectrophotometer for characterization.

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