documentclass{article}
usepackage{caption}
usepackage{etoolbox}
usepackage{xstring}

makeatletter
newcommandformatlabel[1]{%
    noexpandarg
    IfSubStr{#1}{.}{%
      StrBefore{#1}{.}[firstcaption]%
      StrBehind{#1}{.}[secondcaption]%
      textbf{firstcaption.} secondcaption}{%
      #1}%
      }
patchcmd{@caption}{#3}{formatlabel{#3}}
makeatother

usepackage{hyperref}

begin{document}

begin{figure}[tb]
    caption{First sentence that should be bold. Second sentence should not be.}
    label{fig:figure1}
end{figure}

end{document}