structural nanotechnology

How to Characterize Gold Nanoparticles’ Surface?

Guest post by Elena Colangelo

Our Topical Review on the characterization of gold nanoparticles (GNPs) has just been published in the Bionconjugate Chemistry Special Issue “Interfacing Inorganic Nanoparticles with Biology”.

The literature is abounding in works on GNPs for applications in biology, catalysis and sensing, among others. GNPs’ appeal resides in their optical properties, together with the well-developed methods of synthesis available and the possibility of functionalizing their surface with small molecules of interest, which can readily self-assemble on the GNPs’ surface forming a monolayer.

However, allegedly the structure and organization of self-assembled monolayers (SAMs) at the GNPs’ surface are in fact aspects too often neglected [though surely not on this blog; RL]. Such elucidation is challenging experimentally, but it is crucial not only to ensure reproducibility, but also to design nanosystems with defined (bio)physicochemical and structural properties, which could then be envisioned to assemble in more complex systems from a “bottom-up” approach.

Our Topical Review gives an overview of the current knowledge and methods available to characterize the GNPs’ surface with different molecular details.

capture

Cartoon illustrating the different levels of GNPs’ surface characterization discussed in the Topical Review.

First, the experimental methods commonly used to provide the basic characterization of functionalized GNPs, such as identification and quantification of the ligands within the monolayer, are detailed with the aid of some examples.

Second, the experimental methods providing information on the monolayer thickness and compactness are reviewed.

Third, considering that the SAM’s thickness and compactness do not only depend on the amount of ligands within the monolayer, but also on their conformation, the experimental methods that can provide such insights are recapitulated. However, we also stressed on the limitations intrinsic to these methods and on the challenges associated to the determination of the structure of SAMs on GNPs.

Fourth, we summarized some of the approaches used to give insights into the organization of different ligands within a SAM. Indeed, mixed SAMs on GNPs are useful since they can impart to the nanoparticles different functionalities and offer a way to tune their stability.

Fifth, highlighting again the limited insights into the SAM’s structure and organization that can be gathered with experimental techniques, we detailed some examples where a combination of experimental and computational approaches was able to provide a compelling description of the system and to assess molecular details that could not have been revealed experimentally.

Overall, this Topical Review gives emphasis on the importance of GNPs’ surface characterization and on fact that even though a number of experimental techniques are available, they are intrinsically limited and they cannot provide a fully detailed picture. Hence, it is advantageous to combine experimental and theoretical approaches to design nanoparticles with desired (bio)physicochemical properties [such as, e.g., our paper under review, currently available as a preprint; RL].

Big tussle over tiny particles, by Lauren K. Wolf , C&EN

Lauren K. Wolf has written a nice overview of the stripy nanoparticle controversy for Chemical & Engineering News, the weekly magazine published by the American Chemical Society. It starts like this:

AS TRUTH SEEKERS, scientists often challenge one another’s work and debate over the details. At the first-ever international scientific conference, for instance, leading chemists argued vociferously over how to define a molecule’s formula. A lot of very smart people at the meeting, held in Germany in 1860, insisted that water was
OH, while others fought for H 2 O.

That squabble might seem tame compared with a dispute that’s been raging
in the nanoscience community during the past decade. […]

Read it all here… if you have access. If you don’t, email me and I will send you a pdf.

 

Nanoparticles for Imaging, Sensing, and Therapeutic Intervention

tocThat is the title of Bogart et al Nano Focus article published yesterday in ACS Nano.

Abstract:

Nanoparticles have the potential to contribute to new modalities in molecular imaging and sensing as well as in therapeutic interventions. In this Nano Focus article, we identify some of the current challenges and knowledge gaps that need to be confronted to accelerate the developments of various applications. Using specific examples, we journey from the characterization of these complex hybrid nanomaterials; continue with surface design and (bio)physicochemical properties, their fate in biological media and cells, and their potential for cancer treatment; and finally reflect on the role of animal models to predict their behavior in humans.

The first discussions about this paper took place during the European Materials Research Society meeting in Strasbourg last year (where several of the authors co-chaired symposium Q).

The power of images & cartoons: Stripy NP, 2004-2013

In his latest Materials Views column entitled “Nanochemistry Reproducibility”, Geoffrey Ozin offers a strongly worded and rather devastating view of scientific standards in nanoscience and makes recommendation for future improvements. Read it all here.

His concluding “Nano Food for Thought” is as follows:

On a final note in the context of nano reproducibility, how does the nano community judge scientific quality? Some might say that the work with amazing images and routine science is looked upon more favorably than the work with amazing science and routine images. High quality images cannot be a substitute for high quality science. It should be science first and photography second! The question is, how representative are these art nano images of your pet nanomaterial and the reproducibility of the synthesis. […]

Geoffrey Ozin does not give any particular example. To my knowledge, he has not expressed any judgement regarding the stripy nanoparticle articles.

Stripy nanoparticles, 2004-2013; 2004 images are reproduced from Jackson et al, Nature Materials, 3, 330 ; 2013 images are reproduced from Ong et al, ACS Nano, 2013 10.1021/nn402414b

Stripy nanoparticles, 2004-2013; 2004 images are reproduced from Jackson et al, Nature Materials, 3, 330 ; 2013 images are reproduced from Ong et al, ACS Nano, 2013 10.1021/nn402414b

 

 

Protein Cages as Theranostic Agent Carriers, Sierin Lim

Please contact me if you would like to meet the speaker

 

Wednesday 6th February, School of Biological Sciences, SR6, 1 pm

Sierin Lim

Division of Bioengineering, School of Chemical and Biomedical Engineering
Nanyang Technological University, 70 Nanyang Drive, Block N1.3, Singapore 637457, SLim@ntu.edu.sg

 

Abstract — Protein cages can be engineered to tailor its function as carriers for therapeutic and diagnostic agents. They are formed by self-assembly of multiple subunits forming hollow spherical cage structures of nanometer size. Due to their proteinaceous nature, the protein cages allow facile modifications on its internal and external surfaces, as well as the subunit interfaces. Modifications on the internal and the external surfaces allow conjugation of small molecule drugs or contrast agent and targeting ligands, respectively. The subunit interaction is of special interest in engineering controlled release property onto the protein cage. Two protein cages, E2 protein and ferritin, are described.

 

Biodata

Sierin Lim obtained both her B.S. and Ph.D. degrees from University of California, Los Angeles (UCLA) in Chemical Engineering and Biomedical Engineering, respectively. She joined Nanyang Technological University (NTU) as Assistant Professor at the end of July 2007 after a 2.5-year postdoctoral research at University of California, Irvine (UCI). She was the Singapore recipient of the 2012 Asia Pacific Research Networking Fellowship from the International Federation for Medical and Biological Engineering.

Dr. Lim’s research focuses on the design, engineering, and development of hybrid nano/microscale devices from biological parts by utilizing protein engineering as a tool. In particular, she is interested in self-assembling protein-derived nanocapsules and photosynthetic biological materials. The project scopes range from understanding the self-assembly mechanism of the nanocapsules and engineering theranostic carriers to the improvement of electron transfer efficiency in a photosynthetic electrochemical cell.

Responding to the response ?

Following the publication of Friday of Stripy Nanoparticles Revisited, one colleague wrote the following to me “I guess now you have to reply to Francesco’s response“.

My hope is that scientists will read carefully both articles, as well as the primary articles, and come to their own conclusions regarding the existence and properties of stripy nanoparticles. Having said that, I am happy to discuss any particular question asked by readers (within time constraints…).

Here, I comment just on one particular point which relates to simple understanding of scanning microscopy and to our first argument, i.e. that the images are not compatible with the stripy hypothesis for basic geometric reasons: if stripes are regularly spaced in 3D they cannot be in a 2D projection.

In the response, the authors address this point as follow:

Hence, the central argument in Lévy’s paper is based on two assumptions: (1) an STM tip moves horizontally on a sample, and (2) tunneling currents flow perfectly vertically from the tip into the substrate holding the sample. Both assumptions are invalid. In reality, the tip follows the contour of the sample. [ 23 ] The correct projection of the sample features being imaged with STM is onto the true tip trajectory, not onto an imaginary flat line. If we assume a tip trajectory that maintains a constant distance from the particle’s center of mass (hence making a semicircular trajectory), then images of the idealized particle shown in ref. [ 1 ] would be projected onto such a semicircle and consequently should show stripes with a spacing of ∼ 1 nm. It should be noted that, were the tip to really move horizontally over the sample, there would be no feedback needed, no feedback loop-artifact possible, and the whole interpretation of the images presented in ref. [ 1 ] (feedback loop artifacts) would be in contradiction with the initial argument

Our argument is not based on the assumption  that “an STM tip moves horizontally on a sample“. Indeed, during a standard STM scan, the feedback loop attempts to keep the tip-sample distance constant and therefore the tip follows the contour. We simply state the obvious, i.e. that an STM image is a 2D projection of a 3D surface. Each line in an STM image represents (as a color) the vertical movement of the tip as a function of the horizontal displacement. Incidentally, if scanning microscopy worked as described in the paragraph quoted above, the image of a square area… would not be square since each line would have different length depending on the topology of the surface.

response to pep

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Phys. Rev. B 61, 2991–2996 (2000)Atomic structure of carbon nanotubes from scanning tunneling microscopy

Venema et al, Fig 3, reproduced from Phys. Rev. B 61, 2991–2996 (2000)
Atomic structure of carbon nanotubes from scanning tunneling microscopy