Fourier Transform Infrared (FTIR) spectroscopy is a powerful analytical technique for characterizing surface functional groups on nanoparticles. By measuring the absorption of infrared radiation, FTIR provides detailed information about molecular vibrations, enabling the identification of specific functional groups such as hydroxyl (-OH), carboxyl (-COOH), and amine (-NH2) moieties. The method is widely used to verify ligand attachment, quantify surface modifications, and distinguish between physisorption and chemisorption processes.
When analyzing nanoparticles, FTIR spectroscopy detects the vibrational modes of bonds present on their surfaces. The technique is sensitive to changes in dipole moments, making it ideal for studying polar functional groups. For example, the stretching vibration of -OH groups typically appears between 3200–3600 cm⁻¹, while -COOH groups exhibit a characteristic C=O stretch near 1700 cm⁻¹ and a broad O-H stretch around 2500–3000 cm⁻¹. Amine groups show N-H stretching vibrations between 3300–3500 cm⁻¹ and bending modes near 1600 cm⁻¹. These spectral markers allow researchers to confirm the presence of surface modifications and assess the success of functionalization procedures.
Verification of ligand attachment involves comparing the FTIR spectra of bare nanoparticles with those of functionalized nanoparticles. New absorption peaks or shifts in existing peaks indicate successful ligand binding. For instance, when gold nanoparticles are functionalized with thiolated ligands, the S-H stretch (2550–2600 cm⁻¹) disappears, confirming covalent bond formation between sulfur and gold. Similarly, silica nanoparticles modified with silane coupling agents show Si-O-Si stretches (1000–1100 cm⁻¹) and additional peaks corresponding to the organic moiety of the silane.
Quantification of surface functional groups can be achieved using calibration curves derived from known concentrations of reference compounds. Peak integration or height measurements of specific vibrational bands are correlated with the amount of functional groups present. For example, the intensity of the C=O stretch (1700 cm⁻¹) in carboxyl-functionalized nanoparticles can be used to estimate surface coverage when compared to standards. However, quantitative analysis requires careful baseline correction and normalization to account for particle size and scattering effects.
Distinguishing between physisorption and chemisorption is critical for understanding the stability of surface modifications. Physisorbed ligands exhibit FTIR spectra similar to their free forms, with minimal peak shifts. In contrast, chemisorbed ligands often show significant shifts or new peaks due to covalent bonding. For example, when polyethylene glycol (PEG) is physisorbed on polymer nanoparticles, its C-O-C stretch (1100 cm⁻¹) remains unchanged. However, if PEG is chemisorbed via a linker, additional peaks or shifts appear, indicating covalent attachment.
Case studies of silica, gold, and polymer nanoparticles highlight the utility of FTIR spectroscopy. Silica nanoparticles functionalized with aminopropyltriethoxysilane (APTES) show distinct N-H stretches (3300–3500 cm⁻¹) and Si-O-Si stretches (1000–1100 cm⁻¹), confirming successful amine modification. Gold nanoparticles coated with citrate exhibit carboxylate stretches (1400 and 1600 cm⁻¹), while thiolate-coated gold nanoparticles lack the S-H stretch, verifying chemisorption. Polymer nanoparticles, such as those made of poly(lactic-co-glycolic acid) (PLGA), display ester C=O stretches (1750 cm⁻¹), and modifications with targeting ligands introduce additional peaks corresponding to the attached molecules.
Specific spectral markers aid in identifying common functionalizations. For silica nanoparticles, the Si-O-Si asymmetric stretch (1050–1100 cm⁻¹) and Si-OH stretch (950 cm⁻¹) are key indicators of surface chemistry. Gold nanoparticles exhibit plasmonic effects that can interfere with FTIR signals, but ligand-specific peaks still provide valuable information. Polymer nanoparticles often show overlapping peaks from their backbone, requiring deconvolution to isolate surface-related signals.
In summary, FTIR spectroscopy is indispensable for characterizing nanoparticle surface functional groups. By analyzing vibrational modes, researchers can verify ligand attachment, quantify modifications, and differentiate between physisorption and chemisorption. Case studies involving silica, gold, and polymer nanoparticles demonstrate the technique's versatility, with specific spectral markers serving as reliable indicators of surface chemistry. The method's sensitivity to molecular vibrations makes it a cornerstone of nanomaterial characterization, enabling precise control over surface properties for diverse applications.