نانوشیمی و نانوتکنولوژی وکاربرد

Since the first demonstration of quantum dots for biological imaging applications in 1998,108, 109 numerous breakthroughs in QDs technology have led to the recent success

of targeted imaging using QDs bothin vitro and   in vivo.110, 111 For example, various cellular components and proteins (in live or fixed cells) have been labeled and visualized

with functionalized QDs, such as the nuclei,¹¹² mitochondria,¹¹³ microtubules,¹¹⁴ actin

filaments,¹¹⁵ cytokeratin, endocytic compartments,¹¹⁶ mortalin,¹¹⁷ and chaperonin  proteins.118-120 The biocompatible QDs have also been used in living mouse, chick and pig models for the mapping of tumors,4, 121, 122 lymph nodes,44, 123-129 vasculature,130-133

and neural system,¹³⁴ etc.However, no QDs have been used in human bodies yet. There have been many serious questions and concerns raised regarding the cytotoxicity of the QDs containing Cd, Se, Te, Hg, and Pb.75, 135-137 These chemicals, especially the heavy-metal elements,

can be potent toxins, neurotoxins, and/or teratogens depending on the dosage,complexation, and accumulation in the liver and the nervous system. Cadmium has a half

life of about 20 years in humans and it is a suspected carcinogen that can accumulate in the liver, kidney, and many other tissues since there is no known active mechanism to

excrete cadmium from the human body.¹³⁸ Although many studies have found no adverse effects of QDs on cell viability, morphology, function, or development over the duration of experiments (hours to days) at concentrations optimized for labeling efficiency,139-142

the cellular toxicity of QDs under extreme conditions such as photo-oxidation and strong UV excitation has been demonstrated,9, 80 In general, the less protected the QD core or  core/shell is, the sooner the appearance of signs of interference with cell viability or

function as a result of Cd²⁺ release. Clearly, the elemental toxicity is a most critical problem that must be addressed before the QDs can be applied for real clinical applications. Extensive scrutiny and research into the toxicity profiles will be needed

before QDs can be employed in any medical procedures. In addition, further studies are needed to investigate the clearance mechanism of QDs from living systems.¹⁴³

Zinc has been considered as a good candidate to replace the heavy metals contained in QDs. Since an average adult ingests 10-15 mg of zinc daily as a nutrient and absorbs about 5 mg,¹⁴⁴ trace amount of zinc is not hazardous to human body. Mn or Cu doped ZnSe QDs have been reported and can cover a similar emission range as that of CdSe QDs.145, 146 Such QDs are less sensitive to environmental changes such as thermal,chemical, and photochemical disturbances. The Cd-free ZnSe_1-xTe_x QDs were also reported with necessary post-preparative UV irradiation at high temperature to show luminescence in the blue region.¹⁴⁷ Evident Technology Inc. developed a type of non-heavy-metal QDs, the T2-MP EviTags,¹⁴⁸ which consist of an InGaP core with ZnS shell,

and have a size of about 25 nm with red emissions at 650-680 nm. Similar InGaP/ZnS core/shell QDs bound to chitosan were demonstrated for deep tissue imaging with the diameter of about 29 nm.¹⁴⁹ Furthermore, pure ZnS can be another good candidate as non-heavy-metal QDs. Many of the ZnS systems were made using the complicated

method with organic solvents,150-152 or deposited in a silica matrix,¹⁵³ which were not suitable for bioimaging applications. Denzler et al.¹⁵⁴ precipitated ZnS colloidal particles in aqueous solution without using capping molecules. As a result, the ZnS particles had large size and settled quickly. Becker et al.¹⁵⁵showed that with the use of a capping molecule, colloidally stable ZnS nanocrystals could be produced in an aqueous process at

pH 8, but no bright emission was obtained from the as-synthesized sample.  Kho et al.¹⁵⁶

reported a colloidal synthesis route of ZnS nanocrystals powder using cysteine as the

capping molecule and in N₂ saturated environment, but didn’t show the PL properties. In another study, the ZnS QDs were produced in water and ethanol solutions with different coatings, showing near-UV emission.37, 157 More recently, the synthesis of ZnS QDs was reported via a two-step reaction at 60^oC also in water and ethanol mixture.158

For human health and environmental considerations, it is important to have bright

stable QDs that are free of heavy metals and are produced through a simple aqueous synthesis route. The purpose of this study is to investigate the aqueous synthesis of non-heavy-metal ZnS QDs with strong visible emissions and excellent stability. In this chapter, the aqueous syntheses of MPA-capped ZnS QDs, MPS-capped ZnS QDs and

MPS-replaced ZnS QDs are described. The btained ZnS QDs were characterized for their morphology and structure. In addition, the photoluminescence properties and stability under various conditions were examined and compared for the QDs prepared

with different capping molecules and methods.