Ion and pH Sensing with Colloidal Nanoparticles: Influence of Surface Charge on Sensing and Colloidal Properties

Abstract

Ion sensors based on colloidal nanoparticles (NPs), either as actively ion-sensing NPs or as nanoscale carrier systems for organic ion-sensing fluorescent chelators typically require a charged surface in order to be colloidally stable. We demonstrate that this surface charge significantly impacts the ion binding and affects the read-out. Sensor read-out should be thus not determined by the bulk ion concentration, but by the local ion concentration in the nano-environment of the NP surface. We present a conclusive model corroborated by experimental data that reproduces the strong distance-dependence of the effect. The experimental data are based on the capability of tuning the distance of a pH-sensitive fluorophore to the surface of NPs in the nanometer (nm) range. This in turn allows for modification of the effective acid dissociation constant value (its logarithmic form, pK(a)) of analyte-sensitive fluorophores by tuning their distance to the underlying colloidal NPs.

Interaction of colloidal nanoparticles with their local environment: the (ionic) nanoenvironment around nanoparticles is different from bulk and determines the physico-chemical properties of the nanoparticles

“…In this way, ligands on the NP surface introduce higher flexibility in achieving ( pH-independent) colloidal stability, in particular via permanently charged ligands and/or ligands providing steric repulsion. As for ligand-free NPs, besides the charge directly associated with the NP surface (now here in particular to the ligand shell), also ligand-coated NPs comprise a diffusive cloud of charge figure 7a, [55]). Please note that these effects are absolutely non-specific and only depend on charge and valency of the ions.…”

“…However, in the case the NP surface is saturated with attached spacer molecules, they all will adopt similar geometry and thus lead to defined distances R. Fluorophores can be linked to the end of the spacer molecules [147,148], and by using spacers with different molecular weight the average distance between fluorophores and the NP surface can be tuned [143,149,150]. This principle has been applied to sense local H þ [55,57] and Cl 2 [56] concentrations. According to figures 7 and 8 for negatively charged NPs, higher H þ and lower Cl 2 concentrations were found close to the NP surface when compared with bulk.…”

Ion transport controlled by nanoparticle-functionalized membranes

“…Previous work has already demonstrated that strong ligand-ligand interactions required for NP film stability are attainable not only for alkanethiol-AuNPs [20][21][22]45,46 , but other core materials such as cobalt oxide and iron oxide with oleylamine and oleic acid ligands 21 , as well as DNA-coated NPs 47 . The end result is a potentially powerful route towards engineering a set of desired interactions in which colloidal NPs are functionalized a priori with the desired functionality 48,49 . In short, ligated NPs serve as fully functionalized building blocks for a type of bottomup assembly for the active layer of membranes.…”

Analytical strategies based on quantum dots for heavy metal ions detection

“…In fact, the development of novel methods based on nanoparticles (NPs) for sensing ions has experienced significant growth during the last decade. [1][2][3][4][5][6][7][8] Among ions, heavy metal ions such as lead, cadmium, and mercury have attracted more attention due to their high toxicity and detrimental health effects. [7][8][9][10][11] Exposure to even very low levels of lead, cadmium, and mercury ions is known to cause cardiovascular diseases, cancer mortality, damage to liver, kidneys, and central nervous system, neurological, reproductive and developmental disorders, which are more serious problems for children particularly.…”

“…In this area, the coupling of ion-sensitive fluorophores to NPs has been of special interest because they can be detected easily using fluorescence microscopy, therefore, allowing the monitoring of dissolved ion concentrations in living cells and organisms. [1][2][3][4][5] Quantum dots (QDs) have been used in spectroscopic detection of metal ions since 2002, 26 and a wide range of ions have been investigated-for both transition metals 27 and heavy metals. 28, 29 However, we focus here on the development of QDbased sensors for the most relevant toxic heavy metal ions, Cd 2þ , Pb 2þ , and Hg 2þ .…”