Heterogeneous Ligand–Nanoparticle Distributions: A Major Obstacle to Scientific Understanding and Commercial Translation

Abstract

CONSPECTUS Nanoparticles conjugated with functional ligands are expected to have a major impact in medicine, photonics, sensing, and nanoarchitecture design. One major obstacle to realizing the promise of these materials, however, is the difficulty in controlling the ligand/nanoparticle ratio. This obstacle can be segmented into three key areas: First, many system designs have failed to account for the true heterogeneity of ligand/nanoparticle ratios that compose each material. Second, the field's accepted level of characterization of ligand-nanoparticle materials is deficient as it does not provide an accurate definition of material composition that is necessary to both understand the material-property relationships as well as to monitor the consistency of the material. In particular, many characterization assays only determine the mean ligand/nanoparticle ratio and do not provide the number and relative amount of the different ligand/nanoparticle ratios that compose a material. Third, some synthetic approaches that are presently in use may not produce consistent material as they are sensitive to reaction kinetics, and the synthetic history of the nanoparticle. In this account we describe the recent advances that we have made in understanding the material composition of ligand-nanoparticle systems. Our work has been enabled by a model system using poly(amidoamine) dendrimers and two small molecule ligands. Using reverse phase high-pressure liquid chromatography (HPLC) we have successfully resolved and quantified the relative amount of each ligand/dendrimer ratio present. This type of information is rare for the entire field of ligand-nanoparticle materials because most analytical techniques have been unable to identify the components in the distribution. Our experimental data indicates that the actual distribution of ligand-nanoparticle components is much more heterogeneous than commonly assumed. The mean ligand/nanoparticle ratio that is so often the only information known about a material is insufficient. This is because the mean does not provide information on the diversity of components in the material and often does not describe the most common component (the mode). Additionally, our experimental data has provided examples of material batches with the same mean ligand/nanoparticle ratio and very different distributions. This discrepancy indicates that the mean cannot be used as the sole metric to assess the reproducibility of a system. We further found that distribution profiles can be highly sensitive to the synthetic history of the starting material as well as slight changes in reaction conditions. We have incorporated the lessons from our experimental data into new systems designs to provide improved control over the ligand/nanoparticle ratios.

Particle-by-Particle Charge Analysis of DNA-Modified Nanoparticles Using Tunable Resistive Pulse Sensing

“…When using nanoparticle systems a mean population zeta potential will not allow the true measure of the ligand distribution across all of the particles to be interpreted, and in a typical reaction the ligand density would follow a poisson distribution [33][34][35] . The spread of the population can have an effect on the reaction kinetics, stability and sensitivity of nanoparticle based assays [36][37][38] .…”

The nature and implications of uniformity in the hierarchical organization of nanomaterials

“…3A). In the low limit, a Poisson distribution can typically be used to describe the number of ligands per nanoparticle, because each ligand attachment occupies insufficient SA to impact subsequent attachments (i.e., attachment events are independent) (49). Given this distribution, an average number of ligands can be a deceptive metric that may not fully describe sample heterogeneity and should be used with caution (46,49).…”

“…In the low limit, a Poisson distribution can typically be used to describe the number of ligands per nanoparticle, because each ligand attachment occupies insufficient SA to impact subsequent attachments (i.e., attachment events are independent) (49). Given this distribution, an average number of ligands can be a deceptive metric that may not fully describe sample heterogeneity and should be used with caution (46,49). In the high limit, the number of ligands can be approximated as dense packing and is dictated by a complex interplay of nanoparticle shape and SA, interligand interactions, and ligand-solvent interactions (10,44).…”

Polymer-Drug Nanoconjugate – An Innovative Nanomedicine: Challenges and Recent Advancements in Rational Formulation Design for Effective Delivery of Poorly Soluble Drugs

“…Therefore it has been difficult to satisfy the drug product quality and regulatory requirements including the metabolic fate of the conjugates relative to traditional formulations. Current literature is replete with research efforts on quantitative techniques to evaluate homogeneity of the drug and ligand distribution in the preparation of PAMAM dendrimers (42,43) and the effect of physical instability (aggregation) of HPMA-folate conjugate on folate receptor-mediated uptake (44). Sophisticated analytical techniques such as small angle neutron scattering (SANS), 2D 1 H NOESY and TOCSY NMR and pulsed gradient NMR have been used to characterize polymer-drug nanoconjugates (45,46).…”