The electron-dispersive X-ray spectroscopy (EDAX) analysis of CuS QDs @ ZnO nanocomposite confirms the presence of the elements Cu, S, Zn, and O in the sample and formation of hybrid nanocomposite as illustrated in Fig. 3b, Table 2. The absence of any other elemental peaks revealed the prepared sample's high purity.
A total of thirty excipients were tested for their potential to react with DNP or FBS to result in the pink or blue colours that indicate the presence of the ARTs. Most of these excipients were selected following expert advice (Dr Pascal Furrer, University of Geneva) from the Handbook of Pharmaceutical Excipients for their likelihood to be used in the formulation of tablets, especially the ones manufactured by direct compression of the material . Furthermore the selected powders are readily available, cheap and can be easily used to manufacture counterfeit tablets. A total of seven excipients produced a yellow or orange reaction using the Fast Red Test from the GPHF-MiniLab® . None of the excipients was found to react with either of the reagents used in our assays and Figure 1 is typical of the results achieved (quality of the photograph shown is poor).
Several analytical techniques using high tech equipment including liquid chromatography-mass spectroscopy, and high performance liquid chromatography coupled with photo diode array detection or electrochemical detection have been developed to detect and quantify artemisinins in formulations or biological fluids. These methods have several merits such as accuracy, repeatability, specificity and precision but they are expensive, require access to sophisticated equipment and expertise and therefore cannot be implemented for routine analysis in the field.
There are three ST analysis types: visual, physical, and chemical, listed here in order of increasing sophistication. Visual analysis may include inspection of packaging integrity, labeling, dosage units, and or various product security features e.g., holograms and microprinting. Physical analysis may involve the evaluation of the disintegration and or dissolution performance and may also employ microscopy, refractometry, and or refractive index measurements. Chemical analysis may include the application of, for example, spectroscopy, spectrometry, chromatography, and or wet chemistry. Although employing all three analysis types improves the confidence of detecting an SF medicine, chemical analysis offers the most direct supporting evidence. However, compared to the other analysis types, chemical analysis, with the exception of wet chemistry, often requires more consideration by the analyst to implement effectively.
X-ray diffraction (XRD) spectroscopy evaluates crystal structures. By measuring the intensity patterns of scattered monochromatic X-rays, the interatomic spacing between chemical bonds can be determined (Kohli and Mittal 2012). These crystal structures can identify drug substances and their various polymorphs as well as medicine coatings and excipient profiles (Gostin and Buckley 2013; Martino et al. 2010; Zou et al. 2018). The only XRD spectrometer commercially available, is the Terra Portable (Olympus Corp). XRD techniques can identify the drug substance but, because of interference from coatings and surface roughness, successful ID generally requires homogenizing the sample (Martino et al. 2010). XRD spectroscopy methods lack sensitivity and is not quantitative (Maurin et al. 2007). Because of the poor performance of this technique and its limited availability, XRD is seldom used for the detection of SF medicines.
Future work should focus on the development of more detailed guidelines for the evaluation of all types of ST analyses. In addition, instrument manufacturers can leverage the proposed USP general chapter and future guidelines to improve their offerings. Incorporating PMS needs and perspectives in product design could also lead to more effective use of vibrational spectroscopy. For example, to better enable the use of vibrational spectroscopy for PMS, medicine manufacturers could consider minimizing the interferences, which impede these technologies. The use of colorants and opaque packaging could, for example, be reduced (Vickers et al. 2018). Although such product decisions are often necessary for ensuring product quality, these decisions can also be influenced by marketing strategies.
AFM force spectroscopy. (A) Sample preparation for AFM force spectroscopy experiments often involves attachment of biomolecules to a substrate surface. Shown here is a streptavidin-biotin sandwich attachment method, in which biotinylated bovine serum albumin (BSA) and streptavidin serve to anchor a biotinylated molecule to a surface. Its interaction partner is attached to the AFM tip and interactions between the two molecules can be monitored from AFM force-distance curves. (B) Schematic AFM force-distance curve.
For measurements of interaction forces or particle elasticity, the molecules of interest are attached to the AFM tip and/or the substrate surface or tethered between tip and surface. For stable attachment of molecules, again the surfaces typically have to be functionalized. Substrate requirements are hence governed by the need to specifically couple or conjugate individual particles to the substrate surface and/or the AFM tip. For example, thiol goups in proteins can form stable sulphur-metal bonds to gold surfaces, while amine groups can be linked to a surface via the bifunctional crosslinker glutaraldehyde or other carboxyl- or aldehyde-based crosslinkers (see above, section Biological conjugation strategies). In addition to chemical functionalization, often biological molecules serve as a stable link to the surface. For instance, the strong interaction of the streptavidin-biotin receptor-ligand system is a popular aid in molecular attachment (see section Biological conjugation strategies and Figures 1 and4). Another popular surface immobilization strategy for AFM force spectroscopy is based on the high affinity and specificity of antigen-antibody systems, as is, for example, exploited for the tethering of digoxygenin end-labeled DNA fragments to anti-digoxygenin coated polystyrene beads in force spectroscopy experiments.
AFM force spectroscopy experiments measure the forces between AFM tip and substrate surface from the degree of bending of the cantilever towards the surface. If no interaction occurs during the time of tip-sample contact between molecules on the tip and those on the surface, no forces are exerted on the cantilever during retraction. In this case, the retraction curve resembles the approach curve (Figure 4B). However, if bonds have developed between molecules on the tip and molecules on the surface or if a molecular tether has formed (or pre-existed) to link tip and substrate surface, a force is exerted on the connection between tip and surface during tip retraction. This force increases until at a critical force, termed the rupture force, breakage of the molecular bonds occurs (Figure 4B). We can hence interpret this rupture force in terms of the strength of an interaction.
In dynamic force spectroscopy (DFS), a dynamic spectrum of bond rupture forces as a function of loading rate is used to map the energy barriers traversed along the force-driven pathway, exposing the differences in energy between barriers. For details on the highly complex approach of DFS, the interested reader is encouraged to refer to one of several excellent, extensive reviews on this topic (for example[67, 68]).
Combination of AFM with other techniques has opened up a wide spectrum of possible applications. These approaches offer insight into sample topography at high resolution from AFM imaging while at the same time providing information on orthogonal sample properties. Because of the resulting additional level of information, these combinatory approaches are referred to as multidimensional techniques. Conjugated systems of nanoparticles and biological molecules are particularly interesting applications for these multidimensional approaches, since the range of accessible sample properties is significantly increased for these hetero-structures. For instance, labeling protein molecules with quantum dots attaches a fluorescent signal to each of the conjugated molecules. The positions of these fluorescent signals can then indicate and identify the positions of the labeled proteins in the context of complex heteromeric assemblies using simultaneous fluorescence microscopy and AFM imaging (Figure 5)[4, 14, 72]. Combined fluorescence and AFM microscopy is conceptually straight forward and achieved by simply placing an AFM on top of an inverted optical microscope equipped for fluorescence imaging. The combinatory system can also be used for simultaneous AFM force spectroscopy and fluorescence approaches. Furthermore, such simultaneous applications allow for further improvement of the time resolution of the experiment, exploiting the higher sampling frequency of fluorescence monitoring. Combined fluorescence-AFM set-ups are now commercially available from a number of different AFM companies.
Other multidimensional applications include combined Raman spectroscopy and AFM[74, 75] or the simultaneous use of AFM as an imaging tool and as a force sensor. This latter approach involves bioconjugation of the AFM imaging probe itself (see also section AFM as a nanorobot to manipulate and assemble bioconjugates) to achieve specific interactions (recognition events) whenever the AFM tip touches the corresponding partner molecule on the surface. Simultaneous AFM t opography and rec ognition imaging (TREC) results in hybrid images containing sample features as well as the locations of the specifically identified molecules in the sample[69, 76].
The power of AFM for the visualization and investigation of bioconjugated nanostructures lies in its high, nanometer resolution capabilities coupled with its ability to image in liquid environment, in which the bioconjugates remain fully functional. The recent advances towards high speed AFM add the invaluable advantage of enhanced time resolution, allowing us to follow many dynamic processes in real time. Furthermore, hybrid AFM applications have demonstrated their unique potentials to simultaneously gain insight on and manipulate bio-nanotechnological constructs. Examples of these are the relatively recent integration of AFM with fluorescence microscopy or combined application of AFM force spectroscopy and topographical imaging. Further advancement and optimization of AFM based platforms with passive observation and/or active manipulation capacities are of great interest for the grand challenge of bioconjugation; to attain an enhanced degree of information on bioconjugated nanoparticles and allow the fine-tuning of bioconjugation to achieve controlled organization of nanostructures. 2b1af7f3a8