A side effect of techniques such as gel filtration or dialysis is sample dilution. Below we discuss two techniques to increase the particle concentration again.
2.10.1 Reverse dialysis
In this technique, the dilute sample is placed in a semi-permeable dialysis bag, which is subsequently covered with dry flakes of poly(ethylene glycol) (PEG). The molecular weight cut-off (MWCO) of the membrane must be chosen such that neither particles nor the PEG polymer can pass through. Therefore, a high molecular weight PEG (e.g. 20 kDa) should be chosen. Continuously check the tubing until the desired concentration is reached.
2.10.2 Ultrafiltration using a centrifugal device
Ultrafiltration uses the centrifugal force to filter a solution over a semi- permeable membrane. A centrifugal device typically consists of a tube with a top reservoir that is separated from a bottom reservoir by a semi-permeable membrane. Initially, the top reservoir holds the sample solution. During cen- trifugation, water and low molecular weight solutes pass through the membrane into the bottom reservoir, while high molecular weight species are retained in the top reservoir. For our experiments we used Nanosep® centrifugal devices with a 10 kDa MWCO Omega™ Membrane.
Chapter 3
Water solubilization of
quantum dots
The water solubilization or phase transfer of QDs has received considerable attention by researchers from the field. This is because the synthesis of cadmium-based QDs normally takes place in the apolar organic solvent phase, while many promising applications require water-compatible QDs. We ex- plored three phase transfer techniques to find out which technique is most suited for our purpose (combining enzymes with colloidal QDs). The evalu- ated phase transfer techniques are ligand exchange with dihydrolipoic acid, encapsulation with poly(styrene-co-maleic anhydride), and micelle encapsula- tion with poly(ethylene glycol)-phospholipids. For each technique, we give the experimental procedure and discuss how the QD properties (e.g. PL quantum yield, hydrodynamic size, ...) are affected by the phase transfer.
The chapter is subdivided as follows. In section 3.1, a general introduction is given to the existing methods for the phase transfer of QDs. Section 3.2 gives the properties of the commercial semiconductor nanocrystals that were used in this work, as well as their PL properties in the apolar solvent phase. Section 3.3 discusses the removal of excess native ligands by repeated precipitation. Sections 3.4, 3.5, and 3.6 describe the phase transfer by ligand exchange, phos- pholipid micelle encapsulation, and polymer wrapping respectively. Finally, in section 3.7, we give an overview of the phase transfer methods and compare them.
3.1 Introduction
The most popular method to prepare Cd-based QDs - the rapid injection of metal and chalcogenide precursors into hot solvent followed by a temperature drop - yields particles that have no intrinsic water solubility. Their surfaces are
3.1. INTRODUCTION| 40
covered with organic ligands with long hydrophobic chains such as alkylamines and tri-n-octylphosphine oxide. Phase transfer from an apolar organic solvent to an aqueous solution can be achieved by functionalization of the QD surface with hydrophilic ligands, for which there are several strategies. Most of these strategies can be classified as either ligand exchange or ligand addition.
3.1.1 Ligand exchange
Ligand exchange means the replacement of the native (hydrophobic) ligands with new bifunctional ligands. The new ligands bear on one side an anchoring group that has a high affinity to bind to the inorganic QD surface and on the opposite side a hydrophilic head group to achieve water compatibility. In fact, the anchoring group should have an higher affinity for the QD surface than the native ligand. Thiols are extensively used in ligand exchange. Deprotonated thiols, thiolates, form strong complexes with many metal ions. The affinity of thiolates for the surfaces of noble and coinage metals is well known [84]. Thiols with a functional head group such as an amine or carboxyl group are readily available. Examples of ligand exchange with a thiol-containing ligand include cysteine [85], cysteamine [45], mercaptoacetic acid [86], dihydrolipoic acid (DHLA) [24], PEG-terminated DHLA [87], DHLA-based zwitterionic li- gands [88], and even alkylthiol-terminated DNA [89]. Ligand exchange with DHLA is schematically represented in figure 3.1.
Thiol ligand exchange has some attractive features. Firstly, there is a large choice of ligands that is available. Secondly, the procedure is easy to execute. Thirdly, the monolayer of ligands does not add much to the particle size. There are however two major drawbacks to thiol ligand exchange. Firstly, the thiolate- metal bond is fairly weak, because it is a dative (dipolar) bond [90]. As a consequence, thiols suffer from a high dynamic dissociation rate [88]. The shelf life of monothiol-capped QDs is therefore limited (~days). This issue is largely resolved by using bidentate thiols. The cooperative binding of the two thiolate groups extends the shelf life from several weeks to years [88,91]. The second and perhaps largest disadvantage is that thiol ligand exchange often causes a reduction of the PL quantum yield [92,93]. This will be further discussed in section 3.4.
3.1.2 Ligand addition
A second strategy is wrapping the hydrophobic QDs with amphiphilic co- polymers. The encapsulation is driven by hydrophobic interactions between the QD native ligands and the hydrophobic parts of the co-polymers. The hydro- philic part of the co-polymers faces the solution side and renders the particles water soluble. Alternatively, the hydrophobic QDs can be buried inside the hy- drophobic core of phospholipids micelles. QDs have been encapsulated by us- ing poly(ethylene glycol)-phospholipid micelles [94,44], poly(styrene-co-maleic
(1)
(2)
(3)
Figure 3.1 – Schematic representation of the organic ligands on the QD sur-
face before and after purification and water solubilization. (1) The traditional purification of QDs by sequential precipitation and redissolution leads to the loss of weakly bound organic ligands from the surface (see section 3.3). The sketch discriminates between weakly bound ligands (in blue) and more strongly bound ligands (in black). (2) Ligand exchange with DHLA replaces the native hydrophobic ligands. Some firmly bound hydrophobic ligands may remain on the surface. (3) PSMA polymer encapsulation retains the native hydrophobic ligands. Hydrophilic groups on the polymer ensure water compatibility.