Disadvantages of quantum dots

    Quantum dots offer several advantages over organic dyes, but quantum dots have their own slew of problems that must be addressed.  Quantum dots can have surface defects which can affect the recombination of electrons and holes by acting as temporary "traps" [4].  This results in blinking of the quantum dots and deteriorates the quantum yield of the dots [4].   Quantum yield is the ratio of emitted to absorption [4].  The blinking effect can be lessened by having a shell around the core [4].  Quantum dots when placed into live cells exhibit aggregation which can interfere with cell function [9].  Also a dilemma exists in trying to get quantum dots inside cells without killing the cells in the delivering process [9].  Although quantum dots are in the nanometer range, biconjugation with different molecules will increase the size of the dots making delivery into cells more difficult [9]. 

    The toxicity of quantum dots to cells is a major issue.  The coating of quantum dots can be cytotoxic such as mercaptoacetic acid [11].  A damaging effect on cells becomes visible at concentrations above ~6μM for mercaptoacetic acid and above 30μM for PEG-silica coated quantum dots [5].   Also if the coating of the quantum dots are compromised, the metallic core itself or the dissolution of the core can be toxic [11].  Cadmium and selenium ions which are used in the core of quantum dots are known to be cytotoxic [5].  In addition, the erosion of the shell may cause undesirable reactions in vivo [11].   Quantum dots may be toxic according to its composition which was seen in in vitro studies [11].  CdTe quantum dots coated with mercaptoacetic acid was found to be toxic to rat pheochromacytoma cells [11].  The mechanism of cell death is unknown but it is believed to be caused by the free Cd present from core degradation [11].  In addition quantum dots cytotoxicity seems to be dose dependent with injection of 5 billion quantum dot micelle into Xenopus blastomeres resulting in cell abnormalities while cells injected with 2 billion micelles showed normal phenotypes [11].  Quantum dot metabolism and degradation within the body is still largely unknown and several studies have shown that quantum dots accumulate in the kidney, spleen, and liver [5].  Also it is unknown whether the quantum dots can be cleared from the body [5].