Surface enhanced Raman spectroscopy (SERS) of amino acids

Pfukwa, Ngaatendwe Buhle Cathrine (2016-03)

Thesis (MSc)--Stellenbosch University, 2016

Thesis

ENGLISH ABSTRACT : Raman spectroscopy (RS) is an invaluable technique for sample identification. This method requires little sample preparation and is not completely non-invasive. The intensity of Raman scattered light can be enormously increased or boosted when a sample molecule is adsorbed on a metallic surface, a technique known as Surface Enhanced Raman spectroscopy (SERS). Since the development of this technique a lot of studies have been done on molecules adsorbed on various types of metallic structures due to the sole purpose of the increase in Raman signal which occurs under such conditions. This has led to the applications of SERS in industry and in basic research. In this study, silver and gold nanospheres of average size 20 nm were successfully synthesised and characterised using UV-Vis (Ultraviolet-visible) spectroscopy and Transmission Electron Microscopy (TEM). Two RS setups were available, a double stage Raman spectrometer using 514.5 nm Ar+ laser as excitation source and a single stage Raman spectrometer using 532 nm frequency doubled Nd:YAG laser as excitation source. The synthesised silver nanospheres were employed in SERS studies on biomolecules (amino acids) using the single stage Raman setup with the aim of advancing SERS as a bio-analytical tool using our in-house developed RS setup. Qualitative analysis was done on amino acid spectra by band profiling and quantitative analysis was performed by carrying out concentration studies so as to determine the detection limit of the measuring instrument. Results are explained based on the setup used and by comparing with what is expected from literature. It was found that amino acids mostly adsorb on a metallic surface via the common carboxylate, amine and R-groups. This is due to the availability of free electron pairs on the oxygen and nitrogen atoms which take part in charge transfer mechanisms and promote chemical enhancement. It was also observed that some amino acids have functional groups which either have strong affinity for metals or have an electronic structure that contribute to chemical enhancement, thus boosting the Raman signal. A low detection limit of 1x10-4 M from amino acid L-Lysine was obtained. Ultimately, these results are new and provide a set of measurements done on four groups of amino acids using gold and two types of silver nanoparticles. These results form a foundation for future studies on larger biological organisations using the setup available in our labs.

AFRIKAANSE OPSOMMING : Raman spektroskopie (RS) is ʼn waardevolle tegniek om onbekende molekules mee uit te ken. Die metode verg baie min voorbereiding van die monster en is meestal nie-indringend. Die intensiteit van die Raman verstrooide lig kan noemenswaardig vergroot of versterk word wanneer die molekule wat ondersoek word geadsorbeer is op ʼn metaal oppervlak, ʼn tegniek wat Oppervlak Versterkte Raman Spektroskopie (SERS) genoem word. Sedert die ontwikkeling van die tegniek, is daar al talle studies gedoen op molekules wat op ʼn groot aantal verskillende metaal strukture geadsorbeer is, met die doel om die Raman sein wat onder die toestande ontstaan te vergroot. Dit het gelei tot toepassings van SERS in industrie en basiese navorsing. In hierdie studie is silwer en goud nano-sfere van gemiddeld 20 nm in deursnee suksesvol gesintetiseer en gekarakteriseer deur middel van UV-Vis (ultraviolet – sigbare) spektroskopie en Transmissie Elektron Mikroskopie (TEM). Twee RS opstellings was beskikbaar; ʼn dubbele rooster Raman spektrometer met ʼn 514.5 nm Ar+ laser as bron en ʼn enkel rooster Raman spektrometer met ʼn 532 nm frekwensie verdubbelde Nd:YAG laser as bron. Die gesintetiseerde silwer nano-sfere was gebruik in SERS meetings op bio-molekules (aminosure) met die enkel rooster Raman opstelling met die doel om SERS te bevorder as ʼn bio-analitiese tegniek deur gebruik te maak van ons tuisgeboude Raman opstelling. Kwalitatiewe analises was op die spektra van die aminosure gedoen deur na die profiele van die Raman bande te kyk, terwyl kwantitatiewe analises gedoen is deur middel van konsentrasie studies om die deteksie limiet van die aminosure op die instrument te bepaal. Die resultate word beskryf in die konteks van die opstelling wat gebruik is en deur hulle te vergelyk met die literatuur. Dit was gevind dat aminosure hoofsaaklik via die karboksilaat, amien, en R-groepe op die metaal oppervlaktes adsorbeer. Dit is weens die beskikbaarheid van vrye elektronpare op die suurstof en stikstof atome wat deel neem aan ladingsuitruil meganismes en sodoende chemiese versterking bevorder. Dit was ook gevind dat van die aminosure funksionele groepe bevat wat of ʼn sterk affiniteit vir metale het, of ʼn elektron struktuur het wat bydrae tot chemiese versterking, en sodoende die Raman sein vergroot. ʼn Lae deteksie limiet van 1x10-4 M was gevind vir L-Lysine. Op die ou end is die resultate nuut en verskaf dit ʼn stel meetings gedoen op vier verskillende groepe aminosure deur gebruik te maak van goue en twee tipes silwer nanopartikels. Hierdie resultate vorm die grondslag vir toekomstige studies op groter biologiese strukture met die bestaande toerusting in ons laboratorium.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98631
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